src

step.go (9698B)

---

 package uniseg
 
 import "unicode/utf8"
 
 // The bit masks used to extract boundary information returned by [Step].
 const (
 	MaskLine     = 3
 	MaskWord     = 4
 	MaskSentence = 8
 )
 
 // The number of bits to shift the boundary information returned by [Step] to
 // obtain the monospace width of the grapheme cluster.
 const ShiftWidth = 4
 
 // The bit positions by which boundary flags are shifted by the [Step] function.
 // These must correspond to the Mask constants.
 const (
 	shiftWord     = 2
 	shiftSentence = 3
 	// shiftwWidth is ShiftWidth above. No mask as these are always the remaining bits.
 )
 
 // The bit positions by which states are shifted by the [Step] function. These
 // values must ensure state values defined for each of the boundary algorithms
 // don't overlap (and that they all still fit in a single int). These must
 // correspond to the Mask constants.
 const (
 	shiftWordState     = 4
 	shiftSentenceState = 9
 	shiftLineState     = 13
 	shiftPropState     = 21 // No mask as these are always the remaining bits.
 )
 
 // The bit mask used to extract the state returned by the [Step] function, after
 // shifting. These values must correspond to the shift constants.
 const (
 	maskGraphemeState = 0xf
 	maskWordState     = 0x1f
 	maskSentenceState = 0xf
 	maskLineState     = 0xff
 )
 
 // Step returns the first grapheme cluster (user-perceived character) found in
 // the given byte slice. It also returns information about the boundary between
 // that grapheme cluster and the one following it as well as the monospace width
 // of the grapheme cluster. There are three types of boundary information: word
 // boundaries, sentence boundaries, and line breaks. This function is therefore
 // a combination of [FirstGraphemeCluster], [FirstWord], [FirstSentence], and
 // [FirstLineSegment].
 //
 // The "boundaries" return value can be evaluated as follows:
 //
 //   - boundaries&MaskWord != 0: The boundary is a word boundary.
 //   - boundaries&MaskWord == 0: The boundary is not a word boundary.
 //   - boundaries&MaskSentence != 0: The boundary is a sentence boundary.
 //   - boundaries&MaskSentence == 0: The boundary is not a sentence boundary.
 //   - boundaries&MaskLine == LineDontBreak: You must not break the line at the
 //     boundary.
 //   - boundaries&MaskLine == LineMustBreak: You must break the line at the
 //     boundary.
 //   - boundaries&MaskLine == LineCanBreak: You may or may not break the line at
 //     the boundary.
 //   - boundaries >> ShiftWidth: The width of the grapheme cluster for most
 //     monospace fonts where a value of 1 represents one character cell.
 //
 // This function can be called continuously to extract all grapheme clusters
 // from a byte slice, as illustrated in the examples below.
 //
 // If you don't know which state to pass, for example when calling the function
 // for the first time, you must pass -1. For consecutive calls, pass the state
 // and rest slice returned by the previous call.
 //
 // The "rest" slice is the sub-slice of the original byte slice "b" starting
 // after the last byte of the identified grapheme cluster. If the length of the
 // "rest" slice is 0, the entire byte slice "b" has been processed. The
 // "cluster" byte slice is the sub-slice of the input slice containing the
 // first identified grapheme cluster.
 //
 // Given an empty byte slice "b", the function returns nil values.
 //
 // While slightly less convenient than using the Graphemes class, this function
 // has much better performance and makes no allocations. It lends itself well to
 // large byte slices.
 //
 // Note that in accordance with [UAX #14 LB3], the final segment will end with
 // a mandatory line break (boundaries&MaskLine == LineMustBreak). You can choose
 // to ignore this by checking if the length of the "rest" slice is 0 and calling
 // [HasTrailingLineBreak] or [HasTrailingLineBreakInString] on the last rune.
 //
 // [UAX #14 LB3]: https://www.unicode.org/reports/tr14/#Algorithm
 func Step(b []byte, state int) (cluster, rest []byte, boundaries int, newState int) {
 	// An empty byte slice returns nothing.
 	if len(b) == 0 {
 		return
 	}
 
 	// Extract the first rune.
 	r, length := utf8.DecodeRune(b)
 	if len(b) <= length { // If we're already past the end, there is nothing else to parse.
 		var prop int
 		if state < 0 {
 			prop = propertyGraphemes(r)
 		} else {
 			prop = state >> shiftPropState
 		}
 		return b, nil, LineMustBreak | (1 << shiftWord) | (1 << shiftSentence) | (runeWidth(r, prop) << ShiftWidth), grAny | (wbAny << shiftWordState) | (sbAny << shiftSentenceState) | (lbAny << shiftLineState) | (prop << shiftPropState)
 	}
 
 	// If we don't know the state, determine it now.
 	var graphemeState, wordState, sentenceState, lineState, firstProp int
 	remainder := b[length:]
 	if state < 0 {
 		graphemeState, firstProp, _ = transitionGraphemeState(state, r)
 		wordState, _ = transitionWordBreakState(state, r, remainder, "")
 		sentenceState, _ = transitionSentenceBreakState(state, r, remainder, "")
 		lineState, _ = transitionLineBreakState(state, r, remainder, "")
 	} else {
 		graphemeState = state & maskGraphemeState
 		wordState = (state >> shiftWordState) & maskWordState
 		sentenceState = (state >> shiftSentenceState) & maskSentenceState
 		lineState = (state >> shiftLineState) & maskLineState
 		firstProp = state >> shiftPropState
 	}
 
 	// Transition until we find a grapheme cluster boundary.
 	width := runeWidth(r, firstProp)
 	for {
 		var (
 			graphemeBoundary, wordBoundary, sentenceBoundary bool
 			lineBreak, prop                                  int
 		)
 
 		r, l := utf8.DecodeRune(remainder)
 		remainder = b[length+l:]
 
 		graphemeState, prop, graphemeBoundary = transitionGraphemeState(graphemeState, r)
 		wordState, wordBoundary = transitionWordBreakState(wordState, r, remainder, "")
 		sentenceState, sentenceBoundary = transitionSentenceBreakState(sentenceState, r, remainder, "")
 		lineState, lineBreak = transitionLineBreakState(lineState, r, remainder, "")
 
 		if graphemeBoundary {
 			boundary := lineBreak | (width << ShiftWidth)
 			if wordBoundary {
 				boundary |= 1 << shiftWord
 			}
 			if sentenceBoundary {
 				boundary |= 1 << shiftSentence
 			}
 			return b[:length], b[length:], boundary, graphemeState | (wordState << shiftWordState) | (sentenceState << shiftSentenceState) | (lineState << shiftLineState) | (prop << shiftPropState)
 		}
 
 		if firstProp == prExtendedPictographic {
 			if r == vs15 {
 				width = 1
 			} else if r == vs16 {
 				width = 2
 			}
 		} else if firstProp != prRegionalIndicator && firstProp != prL {
 			width += runeWidth(r, prop)
 		}
 
 		length += l
 		if len(b) <= length {
 			return b, nil, LineMustBreak | (1 << shiftWord) | (1 << shiftSentence) | (width << ShiftWidth), grAny | (wbAny << shiftWordState) | (sbAny << shiftSentenceState) | (lbAny << shiftLineState) | (prop << shiftPropState)
 		}
 	}
 }
 
 // StepString is like [Step] but its input and outputs are strings.
 func StepString(str string, state int) (cluster, rest string, boundaries int, newState int) {
 	// An empty byte slice returns nothing.
 	if len(str) == 0 {
 		return
 	}
 
 	// Extract the first rune.
 	r, length := utf8.DecodeRuneInString(str)
 	if len(str) <= length { // If we're already past the end, there is nothing else to parse.
 		prop := propertyGraphemes(r)
 		return str, "", LineMustBreak | (1 << shiftWord) | (1 << shiftSentence) | (runeWidth(r, prop) << ShiftWidth), grAny | (wbAny << shiftWordState) | (sbAny << shiftSentenceState) | (lbAny << shiftLineState)
 	}
 
 	// If we don't know the state, determine it now.
 	var graphemeState, wordState, sentenceState, lineState, firstProp int
 	remainder := str[length:]
 	if state < 0 {
 		graphemeState, firstProp, _ = transitionGraphemeState(state, r)
 		wordState, _ = transitionWordBreakState(state, r, nil, remainder)
 		sentenceState, _ = transitionSentenceBreakState(state, r, nil, remainder)
 		lineState, _ = transitionLineBreakState(state, r, nil, remainder)
 	} else {
 		graphemeState = state & maskGraphemeState
 		wordState = (state >> shiftWordState) & maskWordState
 		sentenceState = (state >> shiftSentenceState) & maskSentenceState
 		lineState = (state >> shiftLineState) & maskLineState
 		firstProp = state >> shiftPropState
 	}
 
 	// Transition until we find a grapheme cluster boundary.
 	width := runeWidth(r, firstProp)
 	for {
 		var (
 			graphemeBoundary, wordBoundary, sentenceBoundary bool
 			lineBreak, prop                                  int
 		)
 
 		r, l := utf8.DecodeRuneInString(remainder)
 		remainder = str[length+l:]
 
 		graphemeState, prop, graphemeBoundary = transitionGraphemeState(graphemeState, r)
 		wordState, wordBoundary = transitionWordBreakState(wordState, r, nil, remainder)
 		sentenceState, sentenceBoundary = transitionSentenceBreakState(sentenceState, r, nil, remainder)
 		lineState, lineBreak = transitionLineBreakState(lineState, r, nil, remainder)
 
 		if graphemeBoundary {
 			boundary := lineBreak | (width << ShiftWidth)
 			if wordBoundary {
 				boundary |= 1 << shiftWord
 			}
 			if sentenceBoundary {
 				boundary |= 1 << shiftSentence
 			}
 			return str[:length], str[length:], boundary, graphemeState | (wordState << shiftWordState) | (sentenceState << shiftSentenceState) | (lineState << shiftLineState) | (prop << shiftPropState)
 		}
 
 		if firstProp == prExtendedPictographic {
 			if r == vs15 {
 				width = 1
 			} else if r == vs16 {
 				width = 2
 			}
 		} else if firstProp != prRegionalIndicator && firstProp != prL {
 			width += runeWidth(r, prop)
 		}
 
 		length += l
 		if len(str) <= length {
 			return str, "", LineMustBreak | (1 << shiftWord) | (1 << shiftSentence) | (width << ShiftWidth), grAny | (wbAny << shiftWordState) | (sbAny << shiftSentenceState) | (lbAny << shiftLineState) | (prop << shiftPropState)
 		}
 	}
 }