// 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 sha3

// spongeDirection indicates the direction bytes are flowing through the sponge.
type spongeDirection int

const (
	// spongeAbsorbing indicates that the sponge is absorbing input.
	spongeAbsorbing spongeDirection = iota
	// spongeSqueezing indicates that the sponge is being squeezed.
	spongeSqueezing
)

const (
	// maxRate is the maximum size of the internal buffer. SHAKE-256
	// currently needs the largest buffer.
	maxRate = 168
)

type state struct {
	// Generic sponge components.
	a    [25]uint64 // main state of the hash
	rate int        // the number of bytes of state to use

	// dsbyte contains the "domain separation" bits and the first bit of
	// the padding. Sections 6.1 and 6.2 of [1] separate the outputs of the
	// SHA-3 and SHAKE functions by appending bitstrings to the message.
	// Using a little-endian bit-ordering convention, these are "01" for SHA-3
	// and "1111" for SHAKE, or 00000010b and 00001111b, respectively. Then the
	// padding rule from section 5.1 is applied to pad the message to a multiple
	// of the rate, which involves adding a "1" bit, zero or more "0" bits, and
	// a final "1" bit. We merge the first "1" bit from the padding into dsbyte,
	// giving 00000110b (0x06) and 00011111b (0x1f).
	// [1] http://csrc.nist.gov/publications/drafts/fips-202/fips_202_draft.pdf
	//     "Draft FIPS 202: SHA-3 Standard: Permutation-Based Hash and
	//      Extendable-Output Functions (May 2014)"
	dsbyte byte

	i, n    int // storage[i:n] is the buffer, i is only used while squeezing
	storage [maxRate]byte

	// Specific to SHA-3 and SHAKE.
	outputLen int             // the default output size in bytes
	state     spongeDirection // whether the sponge is absorbing or squeezing
}

// BlockSize returns the rate of sponge underlying this hash function.
func ( *state) () int { return .rate }

// Size returns the output size of the hash function in bytes.
func ( *state) () int { return .outputLen }

// Reset clears the internal state by zeroing the sponge state and
// the buffer indexes, and setting Sponge.state to absorbing.
func ( *state) () {
	// Zero the permutation's state.
	for  := range .a {
		.a[] = 0
	}
	.state = spongeAbsorbing
	.i, .n = 0, 0
}

func ( *state) () *state {
	 := *
	return &
}

// permute applies the KeccakF-1600 permutation. It handles
// any input-output buffering.
func ( *state) () {
	switch .state {
	case spongeAbsorbing:
		// If we're absorbing, we need to xor the input into the state
		// before applying the permutation.
		xorIn(, .storage[:.rate])
		.n = 0
		keccakF1600(&.a)
	case spongeSqueezing:
		// If we're squeezing, we need to apply the permutation before
		// copying more output.
		keccakF1600(&.a)
		.i = 0
		copyOut(, .storage[:.rate])
	}
}

// pads appends the domain separation bits in dsbyte, applies
// the multi-bitrate 10..1 padding rule, and permutes the state.
func ( *state) () {
	// Pad with this instance's domain-separator bits. We know that there's
	// at least one byte of space in d.buf because, if it were full,
	// permute would have been called to empty it. dsbyte also contains the
	// first one bit for the padding. See the comment in the state struct.
	.storage[.n] = .dsbyte
	.n++
	for .n < .rate {
		.storage[.n] = 0
		.n++
	}
	// This adds the final one bit for the padding. Because of the way that
	// bits are numbered from the LSB upwards, the final bit is the MSB of
	// the last byte.
	.storage[.rate-1] ^= 0x80
	// Apply the permutation
	.permute()
	.state = spongeSqueezing
	.n = .rate
	copyOut(, .storage[:.rate])
}

// Write absorbs more data into the hash's state. It panics if any
// output has already been read.
func ( *state) ( []byte) ( int,  error) {
	if .state != spongeAbsorbing {
		panic("sha3: Write after Read")
	}
	 = len()

	for len() > 0 {
		if .n == 0 && len() >= .rate {
			// The fast path; absorb a full "rate" bytes of input and apply the permutation.
			xorIn(, [:.rate])
			 = [.rate:]
			keccakF1600(&.a)
		} else {
			// The slow path; buffer the input until we can fill the sponge, and then xor it in.
			 := .rate - .n
			if  > len() {
				 = len()
			}
			.n += copy(.storage[.n:], [:])
			 = [:]

			// If the sponge is full, apply the permutation.
			if .n == .rate {
				.permute()
			}
		}
	}

	return
}

// Read squeezes an arbitrary number of bytes from the sponge.
func ( *state) ( []byte) ( int,  error) {
	// If we're still absorbing, pad and apply the permutation.
	if .state == spongeAbsorbing {
		.padAndPermute()
	}

	 = len()

	// Now, do the squeezing.
	for len() > 0 {
		 := copy(, .storage[.i:.n])
		.i += 
		 = [:]

		// Apply the permutation if we've squeezed the sponge dry.
		if .i == .rate {
			.permute()
		}
	}

	return
}

// Sum applies padding to the hash state and then squeezes out the desired
// number of output bytes. It panics if any output has already been read.
func ( *state) ( []byte) []byte {
	if .state != spongeAbsorbing {
		panic("sha3: Sum after Read")
	}

	// Make a copy of the original hash so that caller can keep writing
	// and summing.
	 := .clone()
	 := make([]byte, .outputLen, 64) // explicit cap to allow stack allocation
	.Read()
	return append(, ...)
}