package uniseg

import "unicode/utf8"

// FirstLineSegment returns the prefix of the given byte slice after which a
// decision to break the string over to the next line can or must be made,
// according to the rules of Unicode Standard Annex #14. This is used to
// implement line breaking.
//
// Line breaking, also known as word wrapping, is the process of breaking a
// section of text into lines such that it will fit in the available width of a
// page, window or other display area.
//
// The returned "segment" may not be broken into smaller parts, unless no other
// breaking opportunities present themselves, in which case you may break by
// grapheme clusters (using the FirstGraphemeCluster() function to determine the
// grapheme clusters).
//
// The "mustBreak" flag indicates whether you MUST break the line after the
// given segment (true), for example after newline characters, or you MAY break
// the line after the given segment (false).
//
// This function can be called continuously to extract all non-breaking sub-sets
// from a byte slice, as illustrated in the example below.
//
// If you don't know the current state, 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 line segment. If the length of the
// "rest" slice is 0, the entire byte slice "b" has been processed. The
// "segment" byte slice is the sub-slice of the input slice containing the
// identified line segment.
//
// Given an empty byte slice "b", the function returns nil values.
//
// Note that in accordance with UAX #14 LB3, the final segment will end with
// "mustBreak" set to true. You can choose to ignore this by checking if the
// length of the "rest" slice is 0.
//
// Note also that this algorithm may break within grapheme clusters. This is
// addressed in Section 8.2 Example 6 of UAX #14. To avoid this, you can use
// the Step() function instead.
func FirstLineSegment(b []byte, state int) (segment, rest []byte, mustBreak bool, 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.
		return b, nil, true, lbAny // LB3.
	}

	// If we don't know the state, determine it now.
	if state < 0 {
		state, _ = transitionLineBreakState(state, r, b[length:], "")
	}

	// Transition until we find a boundary.
	var boundary int
	for {
		r, l := utf8.DecodeRune(b[length:])
		state, boundary = transitionLineBreakState(state, r, b[length+l:], "")

		if boundary != LineDontBreak {
			return b[:length], b[length:], boundary == LineMustBreak, state
		}

		length += l
		if len(b) <= length {
			return b, nil, true, lbAny // LB3
		}
	}
}

// FirstLineSegmentInString is like FirstLineSegment() but its input and outputs
// are strings.
func FirstLineSegmentInString(str string, state int) (segment, rest string, mustBreak bool, 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.
		return str, "", true, lbAny // LB3.
	}

	// If we don't know the state, determine it now.
	if state < 0 {
		state, _ = transitionLineBreakState(state, r, nil, str[length:])
	}

	// Transition until we find a boundary.
	var boundary int
	for {
		r, l := utf8.DecodeRuneInString(str[length:])
		state, boundary = transitionLineBreakState(state, r, nil, str[length+l:])

		if boundary != LineDontBreak {
			return str[:length], str[length:], boundary == LineMustBreak, state
		}

		length += l
		if len(str) <= length {
			return str, "", true, lbAny // LB3.
		}
	}
}