// Copyright 2020 The OPA Authors.  All rights reserved.
// Use of this source code is governed by an Apache2
// license that can be found in the LICENSE file.

package bundle

import (
	"bytes"
	"crypto/md5"
	"crypto/sha1"
	"crypto/sha256"
	"crypto/sha512"
	"encoding/json"
	"fmt"
	"hash"
	"io"
	"sort"
	"strings"
)

// HashingAlgorithm represents a subset of hashing algorithms implemented in Go
type HashingAlgorithm string

// Supported values for HashingAlgorithm
const (
	MD5       HashingAlgorithm = "MD5"
	SHA1      HashingAlgorithm = "SHA-1"
	SHA224    HashingAlgorithm = "SHA-224"
	SHA256    HashingAlgorithm = "SHA-256"
	SHA384    HashingAlgorithm = "SHA-384"
	SHA512    HashingAlgorithm = "SHA-512"
	SHA512224 HashingAlgorithm = "SHA-512-224"
	SHA512256 HashingAlgorithm = "SHA-512-256"
)

// String returns the string representation of a HashingAlgorithm
func (alg HashingAlgorithm) String() string {
	return string(alg)
}

// SignatureHasher computes a signature digest for a file with (structured or unstructured) data and policy
type SignatureHasher interface {
	HashFile(v interface{}) ([]byte, error)
}

type hasher struct {
	h func() hash.Hash // hash function factory
}

// NewSignatureHasher returns a signature hasher suitable for a particular hashing algorithm
func NewSignatureHasher(alg HashingAlgorithm) (SignatureHasher, error) {
	h := &hasher{}

	switch alg {
	case MD5:
		h.h = md5.New
	case SHA1:
		h.h = sha1.New
	case SHA224:
		h.h = sha256.New224
	case SHA256:
		h.h = sha256.New
	case SHA384:
		h.h = sha512.New384
	case SHA512:
		h.h = sha512.New
	case SHA512224:
		h.h = sha512.New512_224
	case SHA512256:
		h.h = sha512.New512_256
	default:
		return nil, fmt.Errorf("unsupported hashing algorithm: %s", alg)
	}

	return h, nil
}

// HashFile hashes the file content, JSON or binary, both in golang native format.
func (h *hasher) HashFile(v interface{}) ([]byte, error) {
	hf := h.h()
	walk(v, hf)
	return hf.Sum(nil), nil
}

// walk hashes the file content, JSON or binary, both in golang native format.
//
// Computation for unstructured documents is a hash of the document.
//
// Computation for the types of structured JSON document is as follows:
//
// object: Hash {, then each key (in alphabetical order) and digest of the value, then comma (between items) and finally }.
//
// array: Hash [, then digest of the value, then comma (between items) and finally ].
func walk(v interface{}, h io.Writer) {

	switch x := v.(type) {
	case map[string]interface{}:
		h.Write([]byte("{"))

		var keys []string
		for k := range x {
			keys = append(keys, k)
		}
		sort.Strings(keys)

		for i, key := range keys {
			if i > 0 {
				h.Write([]byte(","))
			}

			h.Write(encodePrimitive(key))
			h.Write([]byte(":"))
			walk(x[key], h)
		}

		h.Write([]byte("}"))
	case []interface{}:
		h.Write([]byte("["))

		for i, e := range x {
			if i > 0 {
				h.Write([]byte(","))
			}
			walk(e, h)
		}

		h.Write([]byte("]"))
	case []byte:
		h.Write(x)
	default:
		h.Write(encodePrimitive(x))
	}
}

func encodePrimitive(v interface{}) []byte {
	var buf bytes.Buffer
	encoder := json.NewEncoder(&buf)
	encoder.SetEscapeHTML(false)
	encoder.Encode(v)
	return []byte(strings.Trim(string(buf.Bytes()), "\n"))
}
