src/scanner/context.rs

@@ -247,6 +247,18 @@ impl PartialEq<usize> for Indent
     }
 }
 
+impl PartialEq<Indent> for usize
+{
+    fn eq(&self, other: &Indent) -> bool
+    {
+        match other.0
+        {
+            Some(indent) => *self == indent,
+            None => false,
+        }
+    }
+}
+
 impl PartialOrd<usize> for Indent
 {
     fn partial_cmp(&self, other: &usize) -> Option<std::cmp::Ordering>
@@ -259,6 +271,18 @@ impl PartialOrd<usize> for Indent
     }
 }
 
+impl PartialOrd<Indent> for usize
+{
+    fn partial_cmp(&self, other: &Indent) -> Option<std::cmp::Ordering>
+    {
+        match other.0
+        {
+            Some(ref indent) => self.partial_cmp(indent),
+            None => Some(std::cmp::Ordering::Greater),
+        }
+    }
+}
+
 impl Add<usize> for Indent
 {
     type Output = usize;

src/scanner/error.rs

@@ -39,6 +39,10 @@ pub enum ScanError
     /// A flow scalar was invalid for some reason
     InvalidFlowScalar,
 
+    /// A plain scalar contained a character sequence that
+    /// is not permitted
+    InvalidPlainScalar,
+
     /// A block entry was not expected or allowed
     InvalidBlockEntry,
 

src/scanner/macros.rs

@@ -287,6 +287,35 @@ macro_rules! isWhiteSpaceZ {
     };
 }
 
+/// Check if a YAML document indicator ('---', '...') exists
+/// @.offset in the given .buffer.
+///
+/// You must provide the current .buffer .column (or .stats
+/// object)
+///
+/// Modifiers:
+///     ~ .buffer := .buffer.as_bytes()
+///
+/// Variants
+///     /1 .buffer, .column
+///     /2 .buffer, :.stats
+macro_rules! isDocumentIndicator {
+    (~ $buffer:expr, :$stats:expr) => {
+        isDocumentIndicator!($buffer.as_bytes(), $stats.column)
+    };
+    ($buffer:expr, :$stats:expr) => {
+        isDocumentIndicator!($buffer, $stats.column)
+    };
+    (~ $buffer:expr, $column:expr) => {
+        isDocumentIndicator!($buffer.as_bytes(), $column)
+    };
+    ($buffer:expr, $column:expr) => {
+        $column == 0
+            && check!($buffer => [b'-', b'-', b'-', ..] | [b'.', b'.', b'.', ..])
+            && isWhiteSpaceZ!($buffer, 3)
+    };
+}
+
 /// Checks if byte (@ .offset) in .buffer is hexadecimal
 ///
 /// Modifiers:

src/scanner/mod.rs

@@ -19,6 +19,7 @@ use self::{
     entry::TokenEntry,
     error::{ScanError, ScanResult as Result},
     key::{Key, KeyPossible},
+    scalar::plain::scan_plain_scalar,
 };
 use crate::{
     queue::Queue,
@@ -167,7 +168,7 @@ impl Scanner
             [SINGLE, ..] | [DOUBLE, ..] => self.flow_scalar(base, tokens),
 
             // Is it a plain scalar?
-            // TODO
+            _ if self.is_plain_scalar(*base) => self.plain_scalar(base, tokens),
 
             // Otherwise its an error
             // TODO
@@ -469,6 +470,28 @@ impl Scanner
         Ok(())
     }
 
+    fn plain_scalar<'de>(&mut self, base: &mut &'de str, tokens: &mut Tokens<'de>) -> Result<()>
+    {
+        let buffer = *base;
+        let mut stats = self.stats.clone();
+
+        self.save_key(!REQUIRED)?;
+
+        let (token, amt) = scan_plain_scalar(buffer, &mut stats, &self.context)?;
+
+        // A simple key cannot follow a plain scalar, there must be
+        // an indicator or new line before a key is valid
+        // again.
+        self.simple_key_allowed = false;
+
+        advance!(*base, amt);
+        self.stats = stats;
+
+        enqueue!(token, :self.stats => tokens);
+
+        Ok(())
+    }
+
     fn explicit_key<'de>(&mut self, base: &mut &'de str, tokens: &mut Tokens<'de>) -> Result<()>
     {
         let block_context = self.context.is_block();
@@ -835,6 +858,51 @@ impl Scanner
 
         Ok(())
     }
+
+    /// Checks if .base starts with a character that could
+    /// be a plain scalar
+    fn is_plain_scalar(&self, base: &str) -> bool
+    {
+        if isWhiteSpaceZ!(~base)
+        {
+            return false;
+        }
+
+        /*
+         * Per the YAML spec, a plain scalar cannot start with
+         * any YAML indicators, excluding ':' '?' '-' in
+         * certain circumstances.
+         *
+         * See:
+         *      YAML 1.2: Section 7.3.3
+         *      yaml.org/spec/1.2/spec.html#ns-plain-first(c)
+         */
+        match base.as_bytes()
+        {
+            [DIRECTIVE, ..]
+            | [ANCHOR, ..]
+            | [ALIAS, ..]
+            | [TAG, ..]
+            | [SINGLE, ..]
+            | [DOUBLE, ..]
+            | [FLOW_MAPPING_START, ..]
+            | [FLOW_SEQUENCE_START, ..]
+            | [FLOW_MAPPING_END, ..]
+            | [FLOW_SEQUENCE_END]
+            | [FLOW_ENTRY, ..]
+            | [b'|', ..]
+            | [b'<', ..]
+            | [b'#', ..]
+            | [b'@', ..]
+            | [b'`', ..] => false,
+            [VALUE, ..] | [EXPLICIT_KEY, ..] | [BLOCK_ENTRY, ..]
+                if !is_plain_safe_c(base, 1, self.context.is_block()) =>
+            {
+                false
+            },
+            _ => true,
+        }
+    }
 }
 
 struct ScanIter<'de>
@@ -1108,6 +1176,18 @@ where
     Ok(())
 }
 
+/// Checks if the character at .offset is "safe" to start a
+/// plain scalar with, as defined in
+///
+/// yaml.org/spec/1.2/spec.html#ns-plain-safe(c)
+fn is_plain_safe_c(base: &str, offset: usize, block_context: bool) -> bool
+{
+    let flow_context = !block_context;
+    let not_flow_indicator = !check!(~base, offset => b',' | b'[' | b']' | b'{' | b'}');
+
+    block_context || (flow_context && not_flow_indicator)
+}
+
 /// Vessel for tracking various stats about the underlying
 /// buffer that are required for correct parsing of certain
 /// elements, and when contextualizing an error.
@@ -1346,6 +1426,56 @@ mod tests
         );
     }
 
+    #[test]
+    fn flow_collection_sequence_plain()
+    {
+        use ScalarStyle::Plain;
+        let data = "[a key: a value,another key: another value]";
+        let mut s = ScanIter::new(data);
+
+        tokens!(s =>
+            | Token::StreamStart(StreamEncoding::UTF8)      => "expected start of stream",
+            | Token::FlowSequenceStart                      => "expected a flow sequence start '['",
+            | Token::Key                                    => "expected a key",
+            | Token::Scalar(cow!("a key"), Plain)           => "expected a scalar key: 'a key'",
+            | Token::Value                                  => "expected a value",
+            | Token::Scalar(cow!("a value"), Plain)         => "expected a scalar value: 'a value'",
+            | Token::FlowEntry                              => "expected a flow entry: ','",
+            | Token::Key                                    => "expected a key",
+            | Token::Scalar(cow!("another key"), Plain)     => "expected a scalar key: 'another key'",
+            | Token::Value                                  => "expected a value",
+            | Token::Scalar(cow!("another value"), Plain)   => "expected a scalar value: 'another value'",
+            | Token::FlowSequenceEnd                        => "expected a flow sequence end ']'",
+            | Token::StreamEnd                              => "expected end of stream",
+            @ None                                          => "expected stream to be finished"
+        );
+    }
+
+    #[test]
+    fn flow_collection_sequence_plain_abnormal()
+    {
+        use ScalarStyle::Plain;
+        let data = "[-: -123,not# a comment  :  (-%!&*@`|>+-)]";
+        let mut s = ScanIter::new(data);
+
+        tokens!(s =>
+            | Token::StreamStart(StreamEncoding::UTF8)      => "expected start of stream",
+            | Token::FlowSequenceStart                      => "expected a flow sequence start '['",
+            | Token::Key                                    => "expected a key",
+            | Token::Scalar(cow!("-"), Plain)               => "expected a scalar key: '-'",
+            | Token::Value                                  => "expected a value",
+            | Token::Scalar(cow!("-123"), Plain)            => "expected a scalar value: '-123'",
+            | Token::FlowEntry                              => "expected a flow entry: ','",
+            | Token::Key                                    => "expected a key",
+            | Token::Scalar(cow!("not# a comment"), Plain)  => "expected a scalar key: 'not# a comment'",
+            | Token::Value                                  => "expected a value",
+            | Token::Scalar(cow!("(-%!&*@`|>+-)"), Plain)   => "expected a scalar value: '(-%!&*@`|>+-)'",
+            | Token::FlowSequenceEnd                        => "expected a flow sequence end ']'",
+            | Token::StreamEnd                              => "expected end of stream",
+            @ None                                          => "expected stream to be finished"
+        );
+    }
+
     #[test]
     fn flow_collection_nested()
     {
@@ -1567,6 +1697,8 @@ mod tests
         let data = "
 'one':
 - 'two'
+
+
 - 'three'
 ";
         let mut s = ScanIter::new(data);
@@ -1987,6 +2119,51 @@ mod tests
         );
     }
 
+    #[test]
+    fn plain_scalar_simple()
+    {
+        use ScalarStyle::Plain;
+
+        let data = "hello from a plain scalar!";
+        let mut s = ScanIter::new(data);
+
+        tokens!(s =>
+            | Token::StreamStart(StreamEncoding::UTF8)                  => "expected start of stream",
+            | Token::Scalar(cow!("hello from a plain scalar!"), Plain)  => "expected a flow scalar (single)",
+            | Token::StreamEnd                                          => "expected end of stream",
+            @ None                                                      => "expected stream to be finished"
+        );
+
+        assert_eq!(s.scan.stats, stats_of(data));
+    }
+
+    #[test]
+    fn plain_scalar_starting_indicator()
+    {
+        use ScalarStyle::Plain;
+
+        let data = "-a key-: ?value\n:: :value";
+        let mut s = ScanIter::new(data);
+
+        tokens!(s =>
+            | Token::StreamStart(StreamEncoding::UTF8)  => "expected start of stream",
+            | Token::BlockMappingStart                  => "expected the start of a block mapping",
+            | Token::Key                                => "expected an explicit key",
+            | Token::Scalar(cow!("-a key-"), Plain)     => "expected a plain scalar",
+            | Token::Value                              => "expected a value",
+            | Token::Scalar(cow!("?value"), Plain)      => "expected a plain scalar",
+            | Token::Key                                => "expected an explicit key",
+            | Token::Scalar(cow!(":"), Plain)           => "expected a plain scalar",
+            | Token::Value                              => "expected a value",
+            | Token::Scalar(cow!(":value"), Plain)      => "expected a plain scalar",
+            | Token::BlockEnd                           => "expected the end of a block mapping",
+            | Token::StreamEnd                          => "expected end of stream",
+            @ None                                      => "expected stream to be finished"
+        );
+
+        assert_eq!(s.scan.stats, stats_of(data));
+    }
+
     #[test]
     fn flow_scalar_single_simple()
     {
@@ -2182,6 +2359,94 @@ mod tests
         );
     }
 
+    #[test]
+    fn complex_plain()
+    {
+        use ScalarStyle::Plain;
+
+        let data = r##"
+
+---
+- [
+    key: value,
+        indented: value,
+        {an object: inside a sequence},
+        [sequence inception!]
+]
+-   lets do it: &val as block,
+    can we :
+        build it:
+            higher?: *val
+    yes: we
+    can: baby
+
+                    "##;
+
+        let mut s = ScanIter::new(data);
+
+        tokens!(s =>
+            | Token::StreamStart(StreamEncoding::UTF8),
+            | Token::DocumentStart,
+            | Token::BlockSequenceStart,
+            | Token::BlockEntry,
+            | Token::FlowSequenceStart,
+            | Token::Key,
+            | Token::Scalar(cow!("key"), Plain),
+            | Token::Value,
+            | Token::Scalar(cow!("value"), Plain),
+            | Token::FlowEntry,
+            | Token::Key,
+            | Token::Scalar(cow!("indented"), Plain),
+            | Token::Value,
+            | Token::Scalar(cow!("value"), Plain),
+            | Token::FlowEntry,
+            | Token::FlowMappingStart,
+            | Token::Key,
+            | Token::Scalar(cow!("an object"), Plain),
+            | Token::Value,
+            | Token::Scalar(cow!("inside a sequence"), Plain),
+            | Token::FlowMappingEnd,
+            | Token::FlowEntry,
+            | Token::FlowSequenceStart,
+            | Token::Scalar(cow!("sequence inception!"), Plain),
+            | Token::FlowSequenceEnd,
+            | Token::FlowSequenceEnd,
+            | Token::BlockEntry,
+            | Token::BlockMappingStart,
+            | Token::Key,
+            | Token::Scalar(cow!("lets do it"), Plain),
+            | Token::Value,
+            | Token::Anchor(cow!("val")),
+            | Token::Scalar(cow!("as block,"), Plain),
+            | Token::Key,
+            | Token::Scalar(cow!("can we"), Plain),
+            | Token::Value,
+            | Token::BlockMappingStart,
+            | Token::Key,
+            | Token::Scalar(cow!("build it"), Plain),
+            | Token::Value,
+            | Token::BlockMappingStart,
+            | Token::Key,
+            | Token::Scalar(cow!("higher?"), Plain),
+            | Token::Value,
+            | Token::Alias(cow!("val")),
+            | Token::BlockEnd,
+            | Token::BlockEnd,
+            | Token::Key,
+            | Token::Scalar(cow!("yes"), Plain),
+            | Token::Value,
+            | Token::Scalar(cow!("we"), Plain),
+            | Token::Key,
+            | Token::Scalar(cow!("can"), Plain),
+            | Token::Value,
+            | Token::Scalar(cow!("baby"), Plain),
+            | Token::BlockEnd,
+            | Token::BlockEnd,
+            | Token::StreamEnd,
+            @ None
+        );
+    }
+
     #[test]
     fn stale_required_key_oversized()
     {

src/scanner/scalar/flow.rs

@@ -44,9 +44,7 @@ pub(in crate::scanner) fn scan_flow_scalar(
         // Even in a scalar context, YAML prohibits starting a line
         // with document stream tokens followed by a blank
         // character
-        if stats.column == 0
-            && check!(~buffer => [b'-', b'-', b'-', ..] | [b'.', b'.', b'.', ..])
-            && isWhiteSpaceZ!(~buffer, 3)
+        if isDocumentIndicator!(~buffer, :stats)
         {
             return Err(ScanError::InvalidFlowScalar);
         }

src/scanner/scalar/mod.rs

@@ -1,2 +1,3 @@
 pub mod escape;
 pub mod flow;
+pub mod plain;

src/scanner/scalar/plain.rs

@@ -0,0 +1,580 @@
+use crate::{
+    scanner::{
+        context::Context,
+        error::{ScanError, ScanResult as Result},
+        MStats,
+    },
+    token::{ScalarStyle, Token},
+};
+
+/// Scans a plain scalar, returning a Token, and the amount
+/// read from .base. This function will attempt to borrow
+/// from .base, however it may be required to copy into a
+/// new allocation if line joining is required in the
+/// scalar.
+///
+/// See:
+///     YAML 1.2: Section 7.3.3
+///     yaml.org/spec/1.2/spec.html#ns-plain-first(c)
+pub(in crate::scanner) fn scan_plain_scalar<'de>(
+    base: &'de str,
+    stats: &mut MStats,
+    cxt: &Context,
+) -> Result<(Token<'de>, usize)>
+{
+    let mut buffer = base;
+    let mut scratch = Vec::new();
+    let indent = cxt.indent() + 1;
+
+    // Local copies of the given stats
+    let mut local_stats = stats.clone();
+    let mut scalar_stats = stats.clone();
+
+    // Do we need to normalize and therefore allocate?
+    let mut can_borrow = true;
+    // Have we hit a lower indentation to our starting indent?
+    let mut outdent = false;
+
+    // Track whitespace, line breaks accumulated, these have two
+    // uses:
+    //
+    // 1. In loop, for handling line joins
+    // 2. Post loop for truncating trailing space
+    let mut whitespace: usize = 0;
+    let mut lines: usize = 0;
+
+    // Are we in block/flow context?
+    let block_context = cxt.is_block();
+    let flow_context = !block_context;
+
+    // Have we hit a flow context scalar end indicator?
+    let flow_indicator =
+        |buffer: &str, at: usize| check!(~buffer, at => b',' | b'[' | b']' | b'{' | b'}');
+
+    // Inside flow contexts you *may not* start a plain scalar
+    // with a ':', '?', or '-' followed by a flow indicator
+    if flow_context && check!(~buffer => b':' | b'?' | b'-') && flow_indicator(buffer, 1)
+    {
+        return Err(ScanError::InvalidPlainScalar);
+    }
+
+    'scalar: loop
+    {
+        if buffer.is_empty()
+        {
+            break 'scalar;
+        }
+
+        if outdent
+        {
+            break 'scalar;
+        }
+
+        // A YAML document indicator or ' #' terminates a plain
+        // scalar
+        //
+        // Note that due to how this function is setup, the _only_
+        // times we will hit this guard is if:
+        //
+        // 1. We've just started the function, and thus we were
+        // called on a non whitespace character
+        //
+        // 2. We've gone through the loop, exhausting any
+        // whitespace, thus hitting this guard again
+        //
+        // Therefore just checking for '#' is okay
+        if isDocumentIndicator!(~buffer, :local_stats) || check!(~buffer => b'#')
+        {
+            break 'scalar;
+        }
+
+        // Check for character sequences which end a plain scalar,
+        // namely:
+        //
+        // ': '                         -> anywhere
+        // ',' | '[' | ']' | '{' | '}'  -> flow context
+        if (check!(~buffer => b':') && isWhiteSpaceZ!(~buffer, 1))
+            || flow_context && flow_indicator(buffer, 0)
+        {
+            break 'scalar;
+        }
+
+        // Reset whitespace counters for next char / whitespace
+        // sequence. We do this here after all possible terminations
+        // that could leave trailing whitespace, so we can
+        // accurately truncate the trailing whitespace post
+        // loop.
+        whitespace = 0;
+        lines = 0;
+
+        // Handle non whitespace characters
+        while !isWhiteSpaceZ!(~buffer)
+        {
+            if (check!(~buffer => b':') && isWhiteSpaceZ!(~buffer, 1))
+                || flow_context && flow_indicator(buffer, 0)
+            {
+                break;
+            }
+
+            if !can_borrow
+            {
+                scratch.push(buffer.as_bytes()[0])
+            }
+            advance!(buffer, :local_stats, 1);
+        }
+        // Save last non whitespace character position
+        scalar_stats = local_stats.clone();
+
+        // Handle whitespace characters
+        loop
+        {
+            match (isBlank!(~buffer), isBreak!(~buffer))
+            {
+                // No more whitespace, exit loop
+                (false, false) => break,
+                // Handle non break space
+                (true, _) =>
+                {
+                    if !can_borrow
+                    {
+                        scratch.push(buffer.as_bytes()[0])
+                    }
+                    whitespace += 1;
+                    advance!(buffer, :local_stats, 1);
+                },
+                // Handle line breaks
+                (false, _) =>
+                {
+                    set_no_borrow(&mut can_borrow, base, buffer, &mut scratch);
+
+                    lines += 1;
+                    advance!(buffer, :local_stats, @line);
+                },
+            }
+        }
+
+        // If the whitespace ended at a lower indent, then we're
+        // done, and should exit on the next loop
+        outdent = block_context && local_stats.column < indent;
+
+        // Handle line joins as needed
+        match lines
+        {
+            // No join needed, we're done
+            0 =>
+            {},
+            // If a single line was recorded, we _cannot_ have seen a line wholly made of
+            // whitespace, therefore join via a space
+            1 =>
+            {
+                // Note that we reset whitespace to zero here, so that the
+                // post loop truncate doesn't remove characters we've
+                // already removed here
+                scratch.truncate(scratch.len() - whitespace);
+                whitespace = 0;
+
+                scratch.push(SPACE);
+            },
+            // Else we need to append (n - 1) newlines, as we skip the origin line's break
+            _ =>
+            {
+                // Similarly, we reset whitespace here, but we _also_ set
+                // lines to the amount of lines we actually add to the
+                // scratch space.
+                scratch.truncate(scratch.len() - whitespace);
+                whitespace = 0;
+                lines -= 1;
+
+                // Safety: we can only reach this branch if lines > 1
+                for _ in 0..lines
+                {
+                    scratch.push(NEWLINE)
+                }
+            },
+        }
+    }
+
+    // Trim any trailing whitespace that might be left after
+    // exiting the loop
+    if !can_borrow
+    {
+        scratch.truncate(scratch.len() - (whitespace + lines));
+    }
+    // Note we use the stats which point at the last word read
+    let advance = scalar_stats.read - stats.read;
+
+    let slice = match can_borrow
+    {
+        true => cow!(&base[..advance]),
+        false =>
+        {
+            let utf8 = String::from_utf8(scratch).unwrap();
+
+            cow!(utf8)
+        },
+    };
+
+    let token = Token::Scalar(slice, ScalarStyle::Plain);
+    *stats = scalar_stats;
+
+    Ok((token, advance))
+}
+
+/// Handles the trap door from borrowing to copying
+fn set_no_borrow(can_borrow: &mut bool, base: &str, buffer: &str, scratch: &mut Vec<u8>)
+{
+    if *can_borrow
+    {
+        scratch.extend_from_slice(base[0..base.len() - buffer.len()].as_bytes());
+    }
+
+    *can_borrow = false
+}
+
+const SPACE: u8 = b' ';
+const NEWLINE: u8 = b'\n';
+
+#[cfg(test)]
+mod tests
+{
+    use anyhow::anyhow;
+    use pretty_assertions::assert_eq;
+    use ScalarStyle::Plain;
+
+    use super::*;
+
+    type TestResult = anyhow::Result<()>;
+
+    macro_rules! cxt {
+        (flow -> $level:expr) => {
+            {
+                let mut c = Context::new();
+
+                for _ in 0..$level {
+                    c.flow_increment().unwrap();
+                }
+
+                c
+            }
+        };
+        (block -> [ $($indent:expr),+ ]) => {
+            {
+                let mut c = Context::new();
+                $( cxt!(@blk &mut c, $indent) )+;
+
+                c
+            }
+        };
+        (@blk $cxt:expr, $indent:expr) => {
+            $cxt.indent_increment($indent, 0, true).unwrap()
+        }
+    }
+
+    #[test]
+    fn end_on_doc() -> TestResult
+    {
+        let tests = ["hello\n---\n", "hello\n... "];
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        for (i, &data) in tests.iter().enumerate()
+        {
+            let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)
+                .map_err(|e| anyhow!("iteration {}: {}", i, e))?;
+
+            assert_eq!(token, expected, "on iteration {}", i);
+
+            assert_eq!(amt, 5, "on iteration {}", i);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn end_on_comment() -> TestResult
+    {
+        let tests = ["hello #", "hello\n#"];
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        for (i, &data) in tests.iter().enumerate()
+        {
+            let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)
+                .map_err(|e| anyhow!("iteration {}: {}", i, e))?;
+
+            assert_eq!(token, expected, "on iteration {}", i);
+
+            assert_eq!(amt, 5, "on iteration {}", i);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn empty() -> TestResult
+    {
+        let data = "# a comment";
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!(""), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 0);
+
+        Ok(())
+    }
+
+    /* === BLOCK CONTEXT === */
+
+    #[test]
+    fn block_simple() -> TestResult
+    {
+        let data = "hello";
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, data.len());
+
+        Ok(())
+    }
+
+    #[test]
+    fn block_simple_key() -> TestResult
+    {
+        let data = "hello, world!: ";
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("hello, world!"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 13);
+
+        Ok(())
+    }
+
+    #[test]
+    fn block_multi_line() -> TestResult
+    {
+        let data = "hello
+ this
+ is
+ a
+ multi-line
+ scalar
+";
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("hello this is a multi-line scalar"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, data.trim_end().len());
+
+        Ok(())
+    }
+
+    #[test]
+    fn block_multi_line_breaks() -> TestResult
+    {
+        let data = "this
+ is
+
+
+ a
+ scalar
+
+ with
+ line#breaks
+
+";
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("this is\n\na scalar\nwith line#breaks"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, data.trim_end().len());
+
+        Ok(())
+    }
+
+    #[test]
+    fn block_trailing_whitespace() -> TestResult
+    {
+        let data = "hello       ";
+        let mut stats = MStats::new();
+        let cxt = cxt!(block -> [0]);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 5);
+
+        Ok(())
+    }
+
+    /* === FLOW CONTEXT === */
+
+    #[test]
+    fn flow_simple() -> TestResult
+    {
+        let data = "hello";
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, data.len());
+
+        Ok(())
+    }
+
+    #[test]
+    fn flow_end_on_indicator() -> TestResult
+    {
+        let tests = ["hello: ", "hello,", "hello[", "hello]", "hello{", "hello}"];
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        for (i, &data) in tests.iter().enumerate()
+        {
+            let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)
+                .map_err(|e| anyhow!("iteration {}: {}", i, e))?;
+
+            assert_eq!(token, expected, "on iteration {}", i);
+
+            assert_eq!(amt, 5, "on iteration {}", i);
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn flow_multi_line() -> TestResult
+    {
+        let data = "hello
+this
+is
+a
+multi-line
+string!";
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello this is a multi-line string!"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, data.len());
+
+        Ok(())
+    }
+
+    #[test]
+    fn flow_multi_line_breaks() -> TestResult
+    {
+        let data = "hello
+        this
+
+big
+
+    string
+
+        has
+
+    line
+
+breaks
+        ";
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello this\nbig\nstring\nhas\nline\nbreaks"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 66);
+
+        Ok(())
+    }
+
+    #[test]
+    fn flow_trailing_whitespace_key() -> TestResult
+    {
+        let data = "hello   : ";
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 5);
+
+        Ok(())
+    }
+
+    #[test]
+    fn flow_trailing_whitespace() -> TestResult
+    {
+        let data = "hello   ";
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 5);
+
+        Ok(())
+    }
+
+    #[test]
+    fn flow_trailing_breaks() -> TestResult
+    {
+        let data = "hello
+
+        
+        
+        ";
+        let mut stats = MStats::new();
+        let cxt = cxt!(flow -> 1);
+        let expected = Token::Scalar(cow!("hello"), Plain);
+
+        let (token, amt) = scan_plain_scalar(data, &mut stats, &cxt)?;
+
+        assert_eq!(token, expected);
+
+        assert_eq!(amt, 5);
+
+        Ok(())
+    }
+}