Steps towards more flexible godoc:

The Mapping object implements a flexible
mapping of relative to absolute paths and
vice versa.

R=rsc
CC=golang-dev
https://golang.org/cl/206067

src/cmd/godoc/mapping.go

+// Copyright 2009 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.
+
+// This file implements the Mapping data structure.
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	pathutil "path"
+	"strings"
+)
+
+
+// A Mapping object maps relative paths (e.g. from URLs)
+// to absolute paths (of the file system) and vice versa.
+//
+// A Mapping object consists of a list of individual mappings
+// of the form: prefix -> path which are interpreted as follows:
+// A relative path of the form prefix/tail is to be mapped to
+// the absolute path/tail, if that absolute path exists in the file
+// system. Given a Mapping object, a relative path is mapped to an
+// absolute path by trying each of the individual mappings in order,
+// until a valid mapping is found. For instance, for the mapping:
+//
+//	user   -> /home/user
+//      public -> /home/user/public
+//	public -> /home/build/public
+//
+// the relative paths below are mapped to absolute paths as follows:
+//
+//	user/foo                -> /home/user/foo
+//      public/net/rpc/file1.go -> /home/user/public/net/rpc/file1.go
+//
+// If there is no /home/user/public/net/rpc/file2.go, the next public
+// mapping entry is used to map the relative path to:
+//
+//	public/net/rpc/file2.go -> /home/build/public/net/rpc/file2.go
+//
+// (assuming that file exists).
+//
+type Mapping struct {
+	list []mapping
+}
+
+
+type mapping struct {
+	prefix, path string
+}
+
+
+// Init initializes the Mapping from a list of ':'-separated
+// paths. Empty paths are ignored; relative paths are assumed
+// to be relative to the current working directory and converted
+// to absolute paths. For each path of the form:
+//
+//	dirname/localname
+//
+// a mapping
+//
+//	localname -> path
+//
+// is added to the Mapping object, in the order of occurrence.
+// For instance, the argument:
+//
+//	/home/user:/home/build/public
+//
+// leads to the following mapping:
+//
+//	user   -> /home/user
+//      public -> /home/build/public
+//
+func (m *Mapping) Init(paths string) {
+	cwd, _ := os.Getwd() // ignore errors
+
+	pathlist := strings.Split(paths, ":", 0)
+
+	list := make([]mapping, len(pathlist))
+	n := 0 // number of mappings
+
+	for _, path := range pathlist {
+		if len(path) == 0 {
+			// ignore empty paths (don't assume ".")
+			continue
+		}
+
+		// len(path) > 0: normalize path
+		if path[0] != '/' {
+			path = pathutil.Join(cwd, path)
+		} else {
+			path = pathutil.Clean(path)
+		}
+
+		// check if mapping exists already
+		var i int
+		for i = 0; i < n; i++ {
+			if path == list[i].path {
+				break
+			}
+		}
+
+		// add mapping if it is new
+		if i >= n {
+			_, prefix := pathutil.Split(path)
+			list[i] = mapping{prefix, path}
+			n++
+		}
+	}
+
+	m.list = list[0:n]
+}
+
+
+// IsEmpty returns true if there are no mappings specified.
+func (m *Mapping) IsEmpty() bool { return len(m.list) == 0 }
+
+
+// Fprint prints the mapping.
+func (m *Mapping) Fprint(w io.Writer) {
+	for _, e := range m.list {
+		fmt.Fprintf(w, "\t%s -> %s\n", e.prefix, e.path)
+	}
+}
+
+
+func split(path string) (head, tail string) {
+	i := strings.Index(path, "/")
+	if i > 0 {
+		// 0 < i < len(path)
+		return path[0:i], path[i+1:]
+	}
+	return "", path
+}
+
+
+// ToAbsolute maps a relative path to an absolute path using the Mapping
+// specified by the receiver. If the path cannot be mapped, the empty
+// string is returned.
+//
+func (m *Mapping) ToAbsolute(path string) string {
+	for _, e := range m.list {
+		if strings.HasPrefix(path, e.path) {
+			// /absolute/prefix/foo -> prefix/foo
+			return pathutil.Join(e.prefix, path[len(e.path):]) // Join will remove a trailing '/'
+		}
+	}
+	return "" // no match
+}
+
+
+// ToRelative maps an absolute path to a relative path using the Mapping
+// specified by the receiver. If the path cannot be mapped, the empty
+// string is returned.
+//
+func (m *Mapping) ToRelative(path string) string {
+	prefix, tail := split(path)
+	for _, e := range m.list {
+		switch {
+		case e.prefix == prefix:
+			// use tail
+		case e.prefix == "":
+			tail = path
+		default:
+			continue // no match
+		}
+		abspath := pathutil.Join(e.path, tail)
+		if _, err := os.Stat(abspath); err == nil {
+			return abspath
+		}
+	}
+
+	return "" // no match
+}