/*
The Moon configuration language.

Purpose

The Moon configuration language is intended to be an alternative to json as
a configuration language for Go projects.  Moon has the following explicit
design goals:

  - to be reasonable for a human to write
  - to be reasonable for a human to read
  - to be reasonable for a machine to generate
  - to be reasonable for a programmer to parse
  - to accomodate documents both large and small

That is, none of these goals is heralded as being the single most important
goal, and Moon makes no claim at being the best format for any of these
individual goals, but it does attempt to consider each of them to at least
some degree.
*/
package moon

import (
	"bytes"
	"encoding/json"
	"fmt"
	"reflect"
	"strconv"
	"strings"
)

// Doc represents a Moon document.
type Doc struct {
	items map[string]interface{}
}

func (d *Doc) MarshalJSON() ([]byte, error) {
	return json.Marshal(d.items)
}

func (d *Doc) MarshalMoon() ([]byte, error) {
	var buf bytes.Buffer
	for k, v := range d.items {
		buf.WriteString(k)
		buf.WriteByte(':')
		buf.WriteByte(' ')
		b, err := Encode(v)
		if err != nil {
			return nil, err
		}
		if _, err := buf.Write(b); err != nil {
			return nil, err
		}
		buf.WriteByte('\n')
	}
	return buf.Bytes(), nil
}

func (d *Doc) Get(path string, dest interface{}) error {
	if d.items == nil {
		return fmt.Errorf("no item found at path %s (doc is empty)", path)
	}

	var v interface{}
	parts := strings.Split(path, "/")

	v, err := seekValue(path, parts, d.items)
	if err != nil {
		return err
	}

	dt := reflect.TypeOf(dest)
	if dt.Kind() != reflect.Ptr {
		return fmt.Errorf("destination is of type %v; a pointer type is required", dt)
	}

	dv := reflect.ValueOf(dest)
	dve := dv.Elem()
	dve.Set(reflect.ValueOf(v))
	return nil
}

func seekValue(fullpath string, parts []string, root interface{}) (interface{}, error) {
	if len(parts) == 0 {
		return nil, fmt.Errorf("path is empty")
	}

	head, tail := parts[0], parts[1:]
	n, err := strconv.Atoi(head)
	if err == nil {
		l, ok := root.([]interface{})
		if !ok {
			return nil, fmt.Errorf("can only index a []interface{}, root is %s", reflect.TypeOf(root))
		}
		if n >= len(l) {
			return nil, fmt.Errorf("path %s is out of bounds, can't get the %d index from a slice of len %d", fullpath, n, len(l))
		}
		v := l[n]
		if len(tail) == 0 {
			return v, nil
		}
		return seekValue(fullpath, tail, v)
	}

	m, ok := root.(map[string]interface{})
	if !ok {
		return nil, fmt.Errorf("can only key a map[string]interface{}, root is %v", reflect.TypeOf(root))
	}

	v, ok := m[head]
	if !ok {
		return nil, fmt.Errorf("unable to seek value at path %s: no value found for part %s", fullpath, head)
	}

	if len(tail) == 0 {
		return v, nil
	}
	return seekValue(fullpath, tail, v)
}