deploytool: Add README.md, migrate docs to it.

deploytool/cmd/param.go

-// Copyright 2016 The LUCI Authors. All rights reserved.
-// Use of this source code is governed under the Apache License, Version 2.0
-// that can be found in the LICENSE file.
-
-package main
-
-import (
-        "regexp"
-        "strings"
-
-        "github.com/luci/luci-go/common/errors"
-)
-
-// paramRE is the regular expression for deployment parameter substitution.
-//
-// Keys can contain alphanumeric letters plus underscores and periods.
-// Substitution is represented as ${KEY}.
-var paramRE = regexp.MustCompile(`\${([a-zA-Z0-9_.]+)}`)
-
-// substitute substitutes any parameter expressions with a value from the
-// supplied substitution map. If a parameter is referenced but not defined,
-// an error will be returned.
-func substitute(vp *string, subs map[string]string) error {
-        var (
-                v       = *vp
-                matches = paramRE.FindAllStringSubmatchIndex(v, -1)
-                parts   = make([]string, 0, (len(matches)*2)+1)
-                sidx    = 0
-        )
-        for _, m := range matches {
-                // m will have 4 entries:
-                // v[m[0]:m[1]] is the variable expression that was matched.
-                // v[m[2]:m[3]] is the paramter key (first capture group)
-                parts = append(parts, v[sidx:m[0]])
-                sidx = m[1]
-
-                key := v[m[2]:m[3]]
-                if val, ok := subs[key]; ok {
-                        parts = append(parts, val)
-                } else {
-                        // panic to immediately stop matching. This will be caught at the top of
-                        // this function.
-                        return errors.Reason("undefined parameter %(key)q").D("key", key).Err()
-                }
-        }
-
-        // Finish the string and return.
-        parts = append(parts, v[sidx:])
-        *vp = strings.Join(parts, "")
-        return nil
-}