package subex

import (
	"main/walk"
)

type RuneReader interface {
	Next() rune
	Rewind()
}

func accept(l RuneReader, chars string) bool {
	r := l.Next()
	for _, char := range chars {
		if char == r {
			return true
		}
	}
	l.Rewind()
	return false
}

func expectBracket(l RuneReader, ifLeft walk.Atom, ifRight walk.Atom) walk.Atom {
	switch l.Next() {
		case '(':
			return ifLeft
		case ')':
			return ifRight
		default:
			panic("Expected ( or )")
	}
}

// Having just read termType, read in a bracket and return the corresponding Atom
func parseTerminatorAtomLiteral(termType rune, l RuneReader) walk.Atom {
	switch termType {
		case '@':
			return expectBracket(l, walk.ArrayBegin, walk.ArrayEnd)
		case '~':
			return expectBracket(l, walk.StartString{}, walk.EndString{})
		case '#':
			return expectBracket(l, walk.MapBegin, walk.MapEnd)
		default:
			return nil
	}
}

func charIsDigit(c rune) bool {
	return '0' <= c && c <= '9'
}

// Parse a positive integer, reads digits 0-9 and stops at the first non-digit
func parseInt(l RuneReader) (output int) {
	for {
		char := l.Next()
		if charIsDigit(char) {
			output = output * 10 + int(char - '0')
		} else {
			break
		}
	}
	l.Rewind()
	return output
}

// Having just read {, read in and parse the range contents
func parseRepeatRange(l RuneReader) (output []ConvexRange) {
	loop: for {
		var start, end int
		char := l.Next()
		l.Rewind()
		if char == '-' {
			start = -1
		} else {
			start = parseInt(l)
		}
		switch l.Next() {
			case ',':
				output = append(output, ConvexRange{start, start})
				continue loop
			case '-':
				char := l.Next()
				if charIsDigit(char) {
					l.Rewind()
					end = parseInt(l)
				} else {
					l.Rewind()
					end = -1
				}
			case '}':
				output = append(output, ConvexRange{start, start})
				break loop
			default:
				panic("Invalid character in repeat specifier")
		}
		switch l.Next() {
			case ',':
				output = append(output, ConvexRange{start, end})
				continue loop
			case '}':
				output = append(output, ConvexRange{start, end})
				break loop
			default:
				panic("Invalid character in repeat specifier")
		}
	}
	return output
}

func parseReplacement(l RuneReader) (output []OutputContent) {
	// TODO escaping
	loop: for {
		r := l.Next()
		switch r {
			case eof:
				panic("Missing closing \"")
			case '"':
				break loop
			case '$':
				slot := l.Next()
				if slot == eof {
					panic("Missing slot character")
				}
				output = append(output, OutputLoad{slot: slot})
			case '@', '~', '#':
				output = append(output, OutputAtomLiteral{atom: parseTerminatorAtomLiteral(r, l)})
			default:
				output = append(output, OutputAtomLiteral{atom: walk.StringAtom(r)})
		}
	}
	return output
}

// Parse the contents of a range subex [] into a map
func parseRangeSubex(l RuneReader) map[walk.Atom]walk.Atom {
	// TODO escaping
	parts := make(map[walk.Atom]walk.Atom)
	var froms []walk.Atom
	var hasTo bool
	for {
		fromsStart := l.Next()
		if fromsStart == ']' {
			hasTo = false
			break
		} else if fromsStart == '=' {
			hasTo = true
			break
		} else {
			atom := parseTerminatorAtomLiteral(fromsStart, l)
			if atom != nil {
				froms = append(froms, atom)
				continue
			}
		}
		if accept(l, "-") {
			fromsEnd := l.Next()
			if fromsEnd == ']' || fromsEnd == '=' {
				l.Rewind()
				fromsEnd = fromsStart
			}
			for i := fromsStart; i <= fromsEnd; i += 1 {
				froms = append(froms, walk.StringAtom(i))
			}
		} else {
			froms = append(froms, walk.StringAtom(fromsStart))
		}
	}
	if len(froms) == 0 {
		panic("Missing from part of range expression")
	}

	var tos []walk.Atom
	if hasTo {
		for {
			tosStart := l.Next()
			if tosStart == ']' {
				break
			} else {
				atom := parseTerminatorAtomLiteral(tosStart, l)
				if atom != nil {
					tos = append(tos, atom)
					continue
				}
			}
			if accept(l, "-") {
				tosEnd := l.Next()
				if tosEnd == ']' {
					l.Rewind()
					tosEnd = tosStart
				}
				for i := tosStart; i <= tosEnd; i += 1 {
					tos = append(tos, walk.StringAtom(i))
				}
			} else {
				tos = append(tos, walk.StringAtom(tosStart))
			}
		}
	} else {
		tos = froms
	}
	if len(tos) == 0 {
		panic("Missing to part of range expression")
	}
	
	for i, from := range froms {
		parts[from] = tos[i % len(tos)]
	}
	return parts
}

func parseSubex(l RuneReader, minPower int) SubexAST {
	var lhs SubexAST
	r := l.Next()
	switch r {
		case eof:
			return nil
		case '(':
			lhs = parseSubex(l, 0)
			if !accept(l, ")") {
				panic("Missing matching )")
			}
		case '[':
			rangeParts := parseRangeSubex(l)
			lhs = SubexASTRange {rangeParts}
		case ')', '|', ';', '{', '+', '$':
			l.Rewind()
			return nil
		case '"':
			replacement := parseReplacement(l)
			lhs = SubexASTOutput{replacement}
		case '.':
			lhs = SubexASTCopyAny{}
		case '@', '#', '~':
			lhs = SubexASTCopyAtom{atom: parseTerminatorAtomLiteral(r, l)}
		default:
			lhs = SubexASTCopyAtom{atom: walk.StringAtom(r)}
	}
	loop: for {
		if minPower <= 0 {
			next := parseSubex(l, 1)
			if next != nil {
				lhs = SubexASTConcat{lhs, next}
				continue loop
			}
		}
		r := l.Next()
		switch {
			case r == '{' && minPower <= 8:
				lhs = SubexASTRepeat {
					content: lhs,
					acceptable: parseRepeatRange(l),
				}
			case r == '+' && minPower <= 8:
				lhs = SubexASTSum {lhs}
			case r == '*' && minPower <= 8:
				lhs = SubexASTProduct {lhs}
			case r == '-' && minPower <= 8:
				lhs = SubexASTNegate {lhs}
			case r == '/' && minPower <= 8:
				lhs = SubexASTReciprocal {lhs}
			case r == '!' && minPower <= 8:
				lhs = SubexASTNot {lhs}
			case r == '$' && minPower <= 8:
				slot := l.Next()
				if slot == eof {
					panic("Missing slot character")
				}
				lhs = SubexASTStore{
					match: lhs,
					slot: slot,
				}
			case r == '|' && minPower <= 4:
				rhs := parseSubex(l, 5)
				if rhs == nil {
					panic("Missing subex after |")
				}
				lhs = SubexASTOr{lhs, rhs}
			case r == ';' && minPower <= 2:
				rhs := parseSubex(l, 3)
				if rhs == nil {
					panic("Missing subex after ;")
				}
				lhs = SubexASTJoin{
					content: lhs,
					delimiter: rhs,
				}
			default:
				l.Rewind()
				break loop
		}
	}
	return lhs
}

func Parse(l RuneReader) SubexAST {
	return parseSubex(l, 0)
}