// Copyright 2017 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.

// Package help implements the ``go help'' command.
package help

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"os"
	"strings"
	"text/template"
	"unicode"
	"unicode/utf8"

	"cmd/go/internal/base"
)

// Help implements the 'help' command.
func Help(args []string) {
	if len(args) == 0 {
		PrintUsage(os.Stdout)
		// not exit 2: succeeded at 'go help'.
		return
	}
	if len(args) != 1 {
		fmt.Fprintf(os.Stderr, "usage: go help command\n\nToo many arguments given.\n")
		os.Exit(2) // failed at 'go help'
	}

	arg := args[0]

	// 'go help documentation' generates doc.go.
	if arg == "documentation" {
		fmt.Println("// Copyright 2011 The Go Authors. All rights reserved.")
		fmt.Println("// Use of this source code is governed by a BSD-style")
		fmt.Println("// license that can be found in the LICENSE file.")
		fmt.Println()
		fmt.Println("// Code generated by mkalldocs.sh; DO NOT EDIT.")
		fmt.Println("// Edit the documentation in other files and rerun mkalldocs.sh to generate this one.")
		fmt.Println()
		buf := new(bytes.Buffer)
		PrintUsage(buf)
		usage := &base.Command{Long: buf.String()}
		cmds := []*base.Command{usage}
		for _, cmd := range base.Commands {
			if cmd.UsageLine == "gopath-get" {
				// Avoid duplication of the "get" documentation.
				continue
			}
			cmds = append(cmds, cmd)
		}
		tmpl(&commentWriter{W: os.Stdout}, documentationTemplate, cmds)
		fmt.Println("package main")
		return
	}

	for _, cmd := range base.Commands {
		if cmd.Name() == arg {
			tmpl(os.Stdout, helpTemplate, cmd)
			// not exit 2: succeeded at 'go help cmd'.
			return
		}
	}

	fmt.Fprintf(os.Stderr, "Unknown help topic %#q. Run 'go help'.\n", arg)
	os.Exit(2) // failed at 'go help cmd'
}

var usageTemplate = `Go is a tool for managing Go source code.

This is vgo, an experimental go command with support for package versioning.
Even though you are invoking it as vgo, most of the messages printed will
still say "go", not "vgo". Sorry.

Usage:

	go command [arguments]

The commands are:
{{range .}}{{if .Runnable}}
	{{.Name | printf "%-11s"}} {{.Short}}{{end}}{{end}}

Use "go help [command]" for more information about a command.

Additional help topics:
{{range .}}{{if not .Runnable}}
	{{.Name | printf "%-11s"}} {{.Short}}{{end}}{{end}}

Use "go help [topic]" for more information about that topic.

`

var helpTemplate = `{{if .Runnable}}usage: go {{.UsageLine}}

{{end}}{{.Long | trim}}
`

var documentationTemplate = `{{range .}}{{if .Short}}{{.Short | capitalize}}

{{end}}{{if .Runnable}}Usage:

	go {{.UsageLine}}

{{end}}{{.Long | trim}}


{{end}}`

// commentWriter writes a Go comment to the underlying io.Writer,
// using line comment form (//).
type commentWriter struct {
	W            io.Writer
	wroteSlashes bool // Wrote "//" at the beginning of the current line.
}

func (c *commentWriter) Write(p []byte) (int, error) {
	var n int
	for i, b := range p {
		if !c.wroteSlashes {
			s := "//"
			if b != '\n' {
				s = "// "
			}
			if _, err := io.WriteString(c.W, s); err != nil {
				return n, err
			}
			c.wroteSlashes = true
		}
		n0, err := c.W.Write(p[i : i+1])
		n += n0
		if err != nil {
			return n, err
		}
		if b == '\n' {
			c.wroteSlashes = false
		}
	}
	return len(p), nil
}

// An errWriter wraps a writer, recording whether a write error occurred.
type errWriter struct {
	w   io.Writer
	err error
}

func (w *errWriter) Write(b []byte) (int, error) {
	n, err := w.w.Write(b)
	if err != nil {
		w.err = err
	}
	return n, err
}

// tmpl executes the given template text on data, writing the result to w.
func tmpl(w io.Writer, text string, data interface{}) {
	t := template.New("top")
	t.Funcs(template.FuncMap{"trim": strings.TrimSpace, "capitalize": capitalize})
	template.Must(t.Parse(text))
	ew := &errWriter{w: w}
	err := t.Execute(ew, data)
	if ew.err != nil {
		// I/O error writing. Ignore write on closed pipe.
		if strings.Contains(ew.err.Error(), "pipe") {
			os.Exit(1)
		}
		base.Fatalf("writing output: %v", ew.err)
	}
	if err != nil {
		panic(err)
	}
}

func capitalize(s string) string {
	if s == "" {
		return s
	}
	r, n := utf8.DecodeRuneInString(s)
	return string(unicode.ToTitle(r)) + s[n:]
}

func PrintUsage(w io.Writer) {
	bw := bufio.NewWriter(w)
	tmpl(bw, usageTemplate, base.Commands)
	bw.Flush()
}