Struct GraphemeCursor

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pub struct GraphemeCursor { /* private fields */ }

Expand description

Cursor-based segmenter for grapheme clusters.

Implementations§

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impl GraphemeCursor

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pub fn new(offset: usize, len: usize) -> GraphemeCursor

Create a new cursor. The string and initial offset are given at creation time, but the contents of the string are not.

The offset parameter must be on a codepoint boundary.

let s = "हिन्दी";
let mut extended = GraphemeCursor::new(0, s.len());
assert_eq!(extended.next_boundary(s, 0), Ok(Some("हि".len())));

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pub fn new_legacy(offset: usize, len: usize) -> GraphemeCursor

Create a new cursor. The string and initial offset are given at creation time, but the contents of the string are not.

The offset parameter must be on a codepoint boundary.

let s = "हिन्दी";
let mut legacy = GraphemeCursor::new_legacy(0, s.len());
assert_eq!(legacy.next_boundary(s, 0), Ok(Some("ह".len())));

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pub fn set_cursor(&mut self, offset: usize)

Set the cursor to a new location in the same string.

let s = "abcd";
let mut cursor = GraphemeCursor::new(0, s.len());
assert_eq!(cursor.cur_cursor(), 0);
cursor.set_cursor(2);
assert_eq!(cursor.cur_cursor(), 2);

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pub fn cur_cursor(&self) -> usize

The current offset of the cursor. Equal to the last value provided to new() or set_cursor(), or returned from next_boundary() or prev_boundary().

// Two flags (🇷🇸🇮🇴), each flag is two RIS codepoints, each RIS is 4 bytes.
let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
let mut cursor = GraphemeCursor::new(4, flags.len());
assert_eq!(cursor.cur_cursor(), 4);
assert_eq!(cursor.next_boundary(flags, 0), Ok(Some(8)));
assert_eq!(cursor.cur_cursor(), 8);

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pub fn provide_context(&mut self, chunk: &str, chunk_start: usize)

Provide additional pre-context when it is needed to decide a boundary. The end of the chunk must coincide with the value given in the GraphemeIncomplete::PreContext request.

let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
let mut cursor = GraphemeCursor::new(8, flags.len());

// Not enough pre-context to decide if there's a boundary between the two flags.
assert_eq!(cursor.is_boundary(&flags[8..], 8), Err(GraphemeIncomplete::PreContext(8)));

// Provide one more Regional Indicator Symbol of pre-context
cursor.provide_context(&flags[4..8], 4);

// Still not enough context to decide.
assert_eq!(cursor.is_boundary(&flags[8..], 8), Err(GraphemeIncomplete::PreContext(4)));

// Provide additional requested context.
cursor.provide_context(&flags[0..4], 0);

// That's enough to decide (it always is when context goes to the start of the string)
assert_eq!(cursor.is_boundary(&flags[8..], 8), Ok(true));

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pub fn is_boundary( &mut self, chunk: &str, chunk_start: usize, ) -> Result<bool, GraphemeIncomplete>

Determine whether the current cursor location is a grapheme cluster boundary. Only a part of the string need be supplied. If chunk_start is nonzero or the length of chunk is not equal to len on creation, then this method may return GraphemeIncomplete::PreContext. The caller should then call provide_context with the requested chunk, then retry calling this method.

For partial chunks, if the cursor is not at the beginning or end of the string, the chunk should contain at least the codepoint following the cursor. If the string is nonempty, the chunk must be nonempty.

All calls should have consistent chunk contents (ie, if a chunk provides content for a given slice, all further chunks covering that slice must have the same content for it).

let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
let mut cursor = GraphemeCursor::new(8, flags.len());
assert_eq!(cursor.is_boundary(flags, 0), Ok(true));
cursor.set_cursor(12);
assert_eq!(cursor.is_boundary(flags, 0), Ok(false));

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pub fn next_boundary( &mut self, chunk: &str, chunk_start: usize, ) -> Result<Option<usize>, GraphemeIncomplete>

Find the next boundary after the current cursor position. Only a part of the string need be supplied. If the chunk is incomplete, then this method might return GraphemeIncomplete::PreContext or GraphemeIncomplete::NextChunk. In the former case, the caller should call provide_context with the requested chunk, then retry. In the latter case, the caller should provide the chunk following the one given, then retry.

See is_boundary for expectations on the provided chunk.

let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
let mut cursor = GraphemeCursor::new(4, flags.len());
assert_eq!(cursor.next_boundary(flags, 0), Ok(Some(8)));
assert_eq!(cursor.next_boundary(flags, 0), Ok(Some(16)));
assert_eq!(cursor.next_boundary(flags, 0), Ok(None));

And an example that uses partial strings:

let s = "abcd";
let mut cursor = GraphemeCursor::new(0, s.len());
assert_eq!(cursor.next_boundary(&s[..2], 0), Ok(Some(1)));
assert_eq!(cursor.next_boundary(&s[..2], 0), Err(GraphemeIncomplete::NextChunk));
assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(Some(2)));
assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(Some(3)));
assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(Some(4)));
assert_eq!(cursor.next_boundary(&s[2..4], 2), Ok(None));

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pub fn prev_boundary( &mut self, chunk: &str, chunk_start: usize, ) -> Result<Option<usize>, GraphemeIncomplete>

Find the previous boundary after the current cursor position. Only a part of the string need be supplied. If the chunk is incomplete, then this method might return GraphemeIncomplete::PreContext or GraphemeIncomplete::PrevChunk. In the former case, the caller should call provide_context with the requested chunk, then retry. In the latter case, the caller should provide the chunk preceding the one given, then retry.

See is_boundary for expectations on the provided chunk.

let flags = "\u{1F1F7}\u{1F1F8}\u{1F1EE}\u{1F1F4}";
let mut cursor = GraphemeCursor::new(12, flags.len());
assert_eq!(cursor.prev_boundary(flags, 0), Ok(Some(8)));
assert_eq!(cursor.prev_boundary(flags, 0), Ok(Some(0)));
assert_eq!(cursor.prev_boundary(flags, 0), Ok(None));

And an example that uses partial strings (note the exact return is not guaranteed, and may be PrevChunk or PreContext arbitrarily):

let s = "abcd";
let mut cursor = GraphemeCursor::new(4, s.len());
assert_eq!(cursor.prev_boundary(&s[2..4], 2), Ok(Some(3)));
assert_eq!(cursor.prev_boundary(&s[2..4], 2), Err(GraphemeIncomplete::PrevChunk));
assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(Some(2)));
assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(Some(1)));
assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(Some(0)));
assert_eq!(cursor.prev_boundary(&s[0..2], 0), Ok(None));