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// Copyright 2011 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 debug contains facilities for programs to debug themselves while
// they are running.
package debug

import (
        "bytes"
        "fmt"
        "io/ioutil"
        "os"
        "runtime"
)

var (
        dunno     = []byte("???")
        centerDot = []byte("·")
        dot       = []byte(".")
)

// PrintStack prints to standard error the stack trace returned by Stack.
func PrintStack() {
        os.Stderr.Write(stack())
}

// Stack returns a formatted stack trace of the goroutine that calls it.
// For each routine, it includes the source line information and PC value,
// then attempts to discover, for Go functions, the calling function or
// method and the text of the line containing the invocation.
func Stack() []byte {
        return stack()
}

// stack implements Stack, skipping 2 frames
func stack() []byte {
        buf := new(bytes.Buffer) // the returned data
        // As we loop, we open files and read them. These variables record the currently
        // loaded file.
        var lines [][]byte
        var lastFile string
        for i := 2; ; i++ { // Caller we care about is the user, 2 frames up
                pc, file, line, ok := runtime.Caller(i)
                if !ok {
                        break
                }
                // Print this much at least.  If we can't find the source, it won't show.
                fmt.Fprintf(buf, "%s:%d (0x%x)\n", file, line, pc)
                if file != lastFile {
                        data, err := ioutil.ReadFile(file)
                        if err != nil {
                                continue
                        }
                        lines = bytes.Split(data, []byte{'\n'})
                        lastFile = file
                }
                line-- // in stack trace, lines are 1-indexed but our array is 0-indexed
                fmt.Fprintf(buf, "\t%s: %s\n", function(pc), source(lines, line))
        }
        return buf.Bytes()
}

// source returns a space-trimmed slice of the n'th line.
func source(lines [][]byte, n int) []byte {
        if n < 0 || n >= len(lines) {
                return dunno
        }
        return bytes.Trim(lines[n], " \t")
}

// function returns, if possible, the name of the function containing the PC.
func function(pc uintptr) []byte {
        fn := runtime.FuncForPC(pc)
        if fn == nil {
                return dunno
        }
        name := []byte(fn.Name())
        // The name includes the path name to the package, which is unnecessary
        // since the file name is already included.  Plus, it has center dots.
        // That is, we see
        //      runtime/debug.*T·ptrmethod
        // and want
        //      *T.ptrmethod
        if period := bytes.Index(name, dot); period >= 0 {
                name = name[period+1:]
        }
        name = bytes.Replace(name, centerDot, dot, -1)
        return name
}