package syntax

import (
	"os"
	"unicode"
)

// A patchList is a list of instruction pointers that need to be filled in (patched).
// Because the pointers haven't been filled in yet, we can reuse their storage
// to hold the list.  It's kind of sleazy, but works well in practice.
// See http://swtch.com/~rsc/regexp/regexp1.html for inspiration.
// 
// These aren't really pointers: they're integers, so we can reinterpret them
// this way without using package unsafe.  A value l denotes
// p.inst[l>>1].Out (l&1==0) or .Arg (l&1==1). 
// l == 0 denotes the empty list, okay because we start every program
// with a fail instruction, so we'll never want to point at its output link.
type patchList uint32

func (l patchList) next(p *Prog) patchList {
	i := &p.Inst[l>>1]
	if l&1 == 0 {
		return patchList(i.Out)
	}
	return patchList(i.Arg)
}

func (l patchList) patch(p *Prog, val uint32) {
	for l != 0 {
		i := &p.Inst[l>>1]
		if l&1 == 0 {
			l = patchList(i.Out)
			i.Out = val
		} else {
			l = patchList(i.Arg)
			i.Arg = val
		}
	}
}

func (l1 patchList) append(p *Prog, l2 patchList) patchList {
	if l1 == 0 {
		return l2
	}
	if l2 == 0 {
		return l1
	}

	last := l1
	for {
		next := last.next(p)
		if next == 0 {
			break
		}
		last = next
	}

	i := &p.Inst[last>>1]
	if last&1 == 0 {
		i.Out = uint32(l2)
	} else {
		i.Arg = uint32(l2)
	}
	return l1
}

// A frag represents a compiled program fragment.
type frag struct {
	i   uint32    // index of first instruction
	out patchList // where to record end instruction
}

type compiler struct {
	p *Prog
}

// Compile compiles the regexp into a program to be executed.
func Compile(re *Regexp) (*Prog, os.Error) {
	var c compiler
	c.init()
	f := c.compile(re)
	f.out.patch(c.p, c.inst(InstMatch).i)
	c.p.Start = int(f.i)
	return c.p, nil
}

func (c *compiler) init() {
	c.p = new(Prog)
	c.inst(InstFail)
}

var anyRuneNotNL = []int{0, '\n' - 1, '\n' - 1, unicode.MaxRune}
var anyRune = []int{0, unicode.MaxRune}

func (c *compiler) compile(re *Regexp) frag {
	switch re.Op {
	case OpNoMatch:
		return c.fail()
	case OpEmptyMatch:
		return c.nop()
	case OpLiteral:
		if len(re.Rune) == 0 {
			return c.nop()
		}
		var f frag
		for j := range re.Rune {
			f1 := c.rune(re.Rune[j : j+1])
			if j == 0 {
				f = f1
			} else {
				f = c.cat(f, f1)
			}
		}
		return f
	case OpCharClass:
		return c.rune(re.Rune)
	case OpAnyCharNotNL:
		return c.rune(anyRuneNotNL)
	case OpAnyChar:
		return c.rune(anyRune)
	case OpBeginLine:
		return c.empty(EmptyBeginLine)
	case OpEndLine:
		return c.empty(EmptyEndLine)
	case OpBeginText:
		return c.empty(EmptyBeginText)
	case OpEndText:
		return c.empty(EmptyEndText)
	case OpWordBoundary:
		return c.empty(EmptyWordBoundary)
	case OpNoWordBoundary:
		return c.empty(EmptyNoWordBoundary)
	case OpCapture:
		bra := c.cap(uint32(re.Cap << 1))
		sub := c.compile(re.Sub[0])
		ket := c.cap(uint32(re.Cap<<1 | 1))
		return c.cat(c.cat(bra, sub), ket)
	case OpStar:
		return c.star(c.compile(re.Sub[0]), re.Flags&NonGreedy != 0)
	case OpPlus:
		return c.plus(c.compile(re.Sub[0]), re.Flags&NonGreedy != 0)
	case OpQuest:
		return c.quest(c.compile(re.Sub[0]), re.Flags&NonGreedy != 0)
	case OpConcat:
		if len(re.Sub) == 0 {
			return c.nop()
		}
		var f frag
		for i, sub := range re.Sub {
			if i == 0 {
				f = c.compile(sub)
			} else {
				f = c.cat(f, c.compile(sub))
			}
		}
		return f
	case OpAlternate:
		var f frag
		for _, sub := range re.Sub {
			f = c.alt(f, c.compile(sub))
		}
		return f
	}
	panic("regexp: unhandled case in compile")
}

func (c *compiler) inst(op InstOp) frag {
	// TODO: impose length limit
	f := frag{i: uint32(len(c.p.Inst))}
	c.p.Inst = append(c.p.Inst, Inst{Op: op})
	return f
}

func (c *compiler) nop() frag {
	f := c.inst(InstNop)
	f.out = patchList(f.i << 1)
	return f
}

func (c *compiler) fail() frag {
	return frag{}
}

func (c *compiler) cap(arg uint32) frag {
	f := c.inst(InstCapture)
	f.out = patchList(f.i << 1)
	c.p.Inst[f.i].Arg = arg
	return f
}

func (c *compiler) cat(f1, f2 frag) frag {
	// concat of failure is failure
	if f1.i == 0 || f2.i == 0 {
		return frag{}
	}

	// TODO: elide nop

	f1.out.patch(c.p, f2.i)
	return frag{f1.i, f2.out}
}

func (c *compiler) alt(f1, f2 frag) frag {
	// alt of failure is other
	if f1.i == 0 {
		return f2
	}
	if f2.i == 0 {
		return f1
	}

	f := c.inst(InstAlt)
	i := &c.p.Inst[f.i]
	i.Out = f1.i
	i.Arg = f2.i
	f.out = f1.out.append(c.p, f2.out)
	return f
}

func (c *compiler) quest(f1 frag, nongreedy bool) frag {
	f := c.inst(InstAlt)
	i := &c.p.Inst[f.i]
	if nongreedy {
		i.Arg = f1.i
		f.out = patchList(f.i << 1)
	} else {
		i.Out = f1.i
		f.out = patchList(f.i<<1 | 1)
	}
	f.out = f.out.append(c.p, f1.out)
	return f
}

func (c *compiler) star(f1 frag, nongreedy bool) frag {
	f := c.inst(InstAlt)
	i := &c.p.Inst[f.i]
	if nongreedy {
		i.Arg = f1.i
		f.out = patchList(f.i << 1)
	} else {
		i.Out = f1.i
		f.out = patchList(f.i<<1 | 1)
	}
	f1.out.patch(c.p, f.i)
	return f
}

func (c *compiler) plus(f1 frag, nongreedy bool) frag {
	return frag{f1.i, c.star(f1, nongreedy).out}
}

func (c *compiler) empty(op EmptyOp) frag {
	f := c.inst(InstEmptyWidth)
	c.p.Inst[f.i].Arg = uint32(op)
	f.out = patchList(f.i << 1)
	return f
}

func (c *compiler) rune(rune []int) frag {
	f := c.inst(InstRune)
	c.p.Inst[f.i].Rune = rune
	f.out = patchList(f.i << 1)
	return f
}