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sectionmap.go
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sectionmap.go
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// Copyright (c) 2018 Timo Savola. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package section
import (
"github.com/tsavola/wag/internal/loader"
"github.com/tsavola/wag/internal/module"
)
const (
moduleHeaderSize = 8
sectionIdSize = 1
)
type ByteRange struct {
Offset int64
Length int64
}
// Map of section positions within the WebAssebly binary module. Map must me
// initialied with MakeMap or NewMap.
//
// Section offset is always nonzero for standard sections; if the section is
// missing, it's the position where it would be. Section length is nonzero if
// the section is present.
//
// Sections[Custom] holds information about the last (or latest) custom
// section. Its offset is zero if there are no custom sections.
type Map struct {
Sections [module.NumSections]ByteRange
}
// MakeMap which represents an empty module.
func MakeMap() (m Map) {
for i := 1; i < int(module.NumSections); i++ {
m.Sections[i].Offset = moduleHeaderSize
}
return
}
// NewMap which represents an empty module.
func NewMap() *Map {
m := MakeMap()
return &m
}
func (m *Map) Mapper() func(byte, Reader) (uint32, error) {
offset := int64(moduleHeaderSize)
return func(sectionId byte, r Reader) (payloadLen uint32, err error) {
payloadLen, payloadLenSize, err := loader.Varuint32(r)
if err != nil {
return
}
length := sectionIdSize + int64(payloadLenSize) + int64(payloadLen)
m.Sections[sectionId] = ByteRange{offset, length}
offset += length
if ID(sectionId) != Custom {
// Default positions of remaining standard sections.
for i := int(sectionId) + 1; i < int(module.NumSections); i++ {
m.Sections[i].Offset = offset
}
}
return
}
}