253 lines
8.2 KiB
Go
253 lines
8.2 KiB
Go
// Copyright (c) 2013-2015 The btcsuite developers
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// Use of this source code is governed by an ISC
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// license that can be found in the LICENSE file.
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package txscript
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import (
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"encoding/binary"
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"fmt"
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)
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const (
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// defaultScriptAlloc is the default size used for the backing array
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// for a script being built by the ScriptBuilder. The array will
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// dynamically grow as needed, but this figure is intended to provide
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// enough space for vast majority of scripts without needing to grow the
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// backing array multiple times.
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defaultScriptAlloc = 500
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)
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// ErrScriptNotCanonical identifies a non-canonical script. The caller can use
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// a type assertion to detect this error type.
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type ErrScriptNotCanonical string
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// Error implements the error interface.
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func (e ErrScriptNotCanonical) Error() string {
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return string(e)
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}
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// ScriptBuilder provides a facility for building custom scripts. It allows
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// you to push opcodes, ints, and data while respecting canonical encoding. In
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// general it does not ensure the script will execute correctly, however any
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// data pushes which would exceed the maximum allowed script engine limits and
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// are therefore guaranteed not to execute will not be pushed and will result in
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// the Script function returning an error.
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//
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// For example, the following would build a 2-of-3 multisig script for usage in
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// a pay-to-script-hash (although in this situation MultiSigScript() would be a
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// better choice to generate the script):
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// builder := txscript.NewScriptBuilder()
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// builder.AddOp(txscript.OP_2).AddData(pubKey1).AddData(pubKey2)
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// builder.AddData(pubKey3).AddOp(txscript.OP_3)
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// builder.AddOp(txscript.OP_CHECKMULTISIG)
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// script, err := builder.Script()
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// if err != nil {
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// // Handle the error.
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// return
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// }
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// fmt.Printf("Final multi-sig script: %x\n", script)
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type ScriptBuilder struct {
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script []byte
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err error
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}
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// AddOp pushes the passed opcode to the end of the script. The script will not
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// be modified if pushing the opcode would cause the script to exceed the
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// maximum allowed script engine size.
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func (b *ScriptBuilder) AddOp(opcode byte) *ScriptBuilder {
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if b.err != nil {
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return b
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}
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// Pushes that would cause the script to exceed the largest allowed
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// script size would result in a non-canonical script.
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if len(b.script)+1 > maxScriptSize {
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str := fmt.Sprintf("adding an opcode would exceed the maximum "+
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"allowed canonical script length of %d", maxScriptSize)
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b.err = ErrScriptNotCanonical(str)
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return b
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}
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b.script = append(b.script, opcode)
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return b
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}
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// canonicalDataSize returns the number of bytes the canonical encoding of the
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// data will take.
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func canonicalDataSize(data []byte) int {
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dataLen := len(data)
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// When the data consists of a single number that can be represented
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// by one of the "small integer" opcodes, that opcode will be instead
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// of a data push opcode followed by the number.
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if dataLen == 0 {
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return 1
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} else if dataLen == 1 && data[0] <= 16 {
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return 1
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} else if dataLen == 1 && data[0] == 0x81 {
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return 1
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}
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if dataLen < OP_PUSHDATA1 {
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return 1 + dataLen
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} else if dataLen <= 0xff {
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return 2 + dataLen
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} else if dataLen <= 0xffff {
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return 3 + dataLen
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}
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return 5 + dataLen
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}
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// addData is the internal function that actually pushes the passed data to the
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// end of the script. It automatically chooses canonical opcodes depending on
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// the length of the data. A zero length buffer will lead to a push of empty
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// data onto the stack (OP_0). No data limits are enforced with this function.
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func (b *ScriptBuilder) addData(data []byte) *ScriptBuilder {
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dataLen := len(data)
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// When the data consists of a single number that can be represented
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// by one of the "small integer" opcodes, use that opcode instead of
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// a data push opcode followed by the number.
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if dataLen == 0 || dataLen == 1 && data[0] == 0 {
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b.script = append(b.script, OP_0)
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return b
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} else if dataLen == 1 && data[0] <= 16 {
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b.script = append(b.script, byte((OP_1-1)+data[0]))
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return b
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} else if dataLen == 1 && data[0] == 0x81 {
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b.script = append(b.script, byte(OP_1NEGATE))
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return b
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}
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// Use one of the OP_DATA_# opcodes if the length of the data is small
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// enough so the data push instruction is only a single byte.
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// Otherwise, choose the smallest possible OP_PUSHDATA# opcode that
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// can represent the length of the data.
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if dataLen < OP_PUSHDATA1 {
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b.script = append(b.script, byte((OP_DATA_1-1)+dataLen))
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} else if dataLen <= 0xff {
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b.script = append(b.script, OP_PUSHDATA1, byte(dataLen))
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} else if dataLen <= 0xffff {
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buf := make([]byte, 2)
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binary.LittleEndian.PutUint16(buf, uint16(dataLen))
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b.script = append(b.script, OP_PUSHDATA2)
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b.script = append(b.script, buf...)
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} else {
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buf := make([]byte, 4)
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binary.LittleEndian.PutUint32(buf, uint32(dataLen))
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b.script = append(b.script, OP_PUSHDATA4)
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b.script = append(b.script, buf...)
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}
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// Append the actual data.
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b.script = append(b.script, data...)
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return b
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}
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// AddFullData should not typically be used by ordinary users as it does not
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// include the checks which prevent data pushes larger than the maximum allowed
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// sizes which leads to scripts that can't be executed. This is provided for
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// testing purposes such as regression tests where sizes are intentionally made
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// larger than allowed.
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//
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// Use AddData instead.
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func (b *ScriptBuilder) AddFullData(data []byte) *ScriptBuilder {
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if b.err != nil {
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return b
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}
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return b.addData(data)
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}
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// AddData pushes the passed data to the end of the script. It automatically
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// chooses canonical opcodes depending on the length of the data. A zero length
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// buffer will lead to a push of empty data onto the stack (OP_0) and any push
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// of data greater than MaxScriptElementSize will not modify the script since
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// that is not allowed by the script engine. Also, the script will not be
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// modified if pushing the data would cause the script to exceed the maximum
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// allowed script engine size.
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func (b *ScriptBuilder) AddData(data []byte) *ScriptBuilder {
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if b.err != nil {
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return b
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}
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// Pushes that would cause the script to exceed the largest allowed
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// script size would result in a non-canonical script.
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dataSize := canonicalDataSize(data)
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if len(b.script)+dataSize > maxScriptSize {
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str := fmt.Sprintf("adding %d bytes of data would exceed the "+
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"maximum allowed canonical script length of %d",
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dataSize, maxScriptSize)
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b.err = ErrScriptNotCanonical(str)
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return b
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}
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// Pushes larger than the max script element size would result in a
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// script that is not canonical.
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dataLen := len(data)
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if dataLen > MaxScriptElementSize {
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str := fmt.Sprintf("adding a data element of %d bytes would "+
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"exceed the maximum allowed script element size of %d",
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dataLen, maxScriptSize)
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b.err = ErrScriptNotCanonical(str)
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return b
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}
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return b.addData(data)
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}
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// AddInt64 pushes the passed integer to the end of the script. The script will
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// not be modified if pushing the data would cause the script to exceed the
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// maximum allowed script engine size.
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func (b *ScriptBuilder) AddInt64(val int64) *ScriptBuilder {
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if b.err != nil {
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return b
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}
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// Pushes that would cause the script to exceed the largest allowed
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// script size would result in a non-canonical script.
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if len(b.script)+1 > maxScriptSize {
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str := fmt.Sprintf("adding an integer would exceed the "+
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"maximum allow canonical script length of %d",
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maxScriptSize)
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b.err = ErrScriptNotCanonical(str)
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return b
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}
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// Fast path for small integers and OP_1NEGATE.
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if val == 0 {
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b.script = append(b.script, OP_0)
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return b
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}
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if val == -1 || (val >= 1 && val <= 16) {
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b.script = append(b.script, byte((OP_1-1)+val))
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return b
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}
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return b.AddData(scriptNum(val).Bytes())
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}
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// Reset resets the script so it has no content.
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func (b *ScriptBuilder) Reset() *ScriptBuilder {
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b.script = b.script[0:0]
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b.err = nil
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return b
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}
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// Script returns the currently built script. When any errors occurred while
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// building the script, the script will be returned up the point of the first
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// error along with the error.
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func (b *ScriptBuilder) Script() ([]byte, error) {
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return b.script, b.err
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}
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// NewScriptBuilder returns a new instance of a script builder. See
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// ScriptBuilder for details.
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func NewScriptBuilder() *ScriptBuilder {
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return &ScriptBuilder{
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script: make([]byte, 0, defaultScriptAlloc),
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}
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}
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