package scrabble

import (
	dawg "github.com/iliadenisov/dafsa"

	"scrabble-solver/board"
	"scrabble-solver/internal/encoding"
)

// letterSet is a bit set over alphabet letter indexes (alphabets are at most 63
// letters, so a uint64 suffices). It encodes a square's cross-set: the letters that,
// placed on the square, form a legal perpendicular word.
type letterSet uint64

func (s letterSet) has(l byte) bool { return s&(letterSet(1)<<l) != 0 }

// fullSet is the cross-set of a square with no perpendicular neighbours: every letter
// is allowed.
func fullSet(size int) letterSet { return letterSet(uint64(1)<<uint(size)) - 1 }

// columnContext returns the contiguous run of filled cells immediately above and below
// the empty square (r, c), each read top to bottom, as alphabet letter indexes. These
// are the tiles a perpendicular (vertical) word through (r, c) would include.
func columnContext(b *board.Board, r, c int) (above, below []byte) {
	start := r
	for start-1 >= 0 && b.Filled(start-1, c) {
		start--
	}
	for rr := start; rr < r; rr++ {
		above = append(above, encoding.Letter(b.At(rr, c)))
	}

	end := r
	for end+1 < b.Rows() && b.Filled(end+1, c) {
		end++
	}
	for rr := r + 1; rr <= end; rr++ {
		below = append(below, encoding.Letter(b.At(rr, c)))
	}
	return above, below
}

// completers returns the letters X (< size) that complete a word when followed from
// state: those whose arc leads directly to an accepting node. It is a single arc
// enumeration — the deterministic cross-set primitive.
func completers(cur *dawg.Cursor, state dawg.Node, size int) letterSet {
	var set letterSet
	lim := byte(size)
	cur.Arcs(state, func(a dawg.Arc) bool {
		if a.Final && a.Label < lim {
			set |= letterSet(1) << a.Label
		}
		return true
	})
	return set
}

// walk follows word left to right from the cursor's root.
func walk(cur *dawg.Cursor, word []byte) (dawg.Node, bool) {
	n := cur.Root()
	for _, l := range word {
		var ok bool
		if n, _, ok = cur.Next(n, l); !ok {
			return n, false
		}
	}
	return n, true
}

// dawgCrossSet returns the letters X for which above·X·below is a stored word. A right
// extension (no tiles below) is deterministic — X just completes the prefix above. A
// left extension (tiles below) is non-deterministic and must probe each X.
func dawgCrossSet(cur *dawg.Cursor, above, below []byte, size int) letterSet {
	switch {
	case len(above) == 0 && len(below) == 0:
		return fullSet(size)
	case len(below) == 0:
		node, ok := walk(cur, above)
		if !ok {
			return 0
		}
		return completers(cur, node, size)
	default:
		node := cur.Root()
		if len(above) > 0 {
			var ok bool
			if node, ok = walk(cur, above); !ok {
				return 0
			}
		}
		var set letterSet
		for x := range size {
			m, final, ok := cur.Next(node, byte(x))
			if !ok {
				continue
			}
			for _, l := range below {
				if m, final, ok = cur.Next(m, l); !ok {
					break
				}
			}
			if ok && final {
				set |= letterSet(1) << uint(x)
			}
		}
		return set
	}
}