internal/triegen/compact.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package triegen

// This file defines Compacter and its implementations.

import "io"

// A Compacter generates an alternative, more space-efficient way to store a
// trie value block. A trie value block holds all possible values for the last
// byte of a UTF-8 encoded rune. Excluding ASCII characters, a trie value block
// always has 64 values, as a UTF-8 encoding ends with a byte in [0x80, 0xC0).
type Compacter interface {
	// Size returns whether the Compacter could encode the given block as well
	// as its size in case it can. len(v) is always 64.
	Size(v []uint64) (sz int, ok bool)

	// Store stores the block using the Compacter's compression method.
	// It returns a handle with which the block can be retrieved.
	// len(v) is always 64.
	Store(v []uint64) uint32

	// Print writes the data structures associated to the given store to w.
	Print(w io.Writer) error

	// Handler returns the name of a function that gets called during trie
	// lookup for blocks generated by the Compacter. The function should be of
	// the form func (n uint32, b byte) uint64, where n is the index returned by
	// the Compacter's Store method and b is the last byte of the UTF-8
	// encoding, where 0x80 <= b < 0xC0, for which to do the lookup in the
	// block.
	Handler() string
}

// simpleCompacter is the default Compacter used by builder. It implements a
// normal trie block.
type simpleCompacter builder

func (b *simpleCompacter) Size([]uint64) (sz int, ok bool) {
	return blockSize * b.ValueSize, true
}

func (b *simpleCompacter) Store(v []uint64) uint32 {
	h := uint32(len(b.ValueBlocks) - blockOffset)
	b.ValueBlocks = append(b.ValueBlocks, v)
	return h
}

func (b *simpleCompacter) Print(io.Writer) error {
	// Structures are printed in print.go.
	return nil
}

func (b *simpleCompacter) Handler() string {
	panic("Handler should be special-cased for this Compacter")
}