// Copyright 2015 Tamás Gulácsi. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the 'License');
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an 'AS IS' BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package olc implements the Open Location Code algorithm to convert latitude and longitude coordinates
// into a shorter sequence of letters and numbers.
//
// The aim is to provide something that can be used like an address in locations that lack them, because
// the streets are unnamed.
//
// Codes represent areas, and the size of the area depends on the length of the code. The typical code
// length is 10 digits, and represents an area of 1/8000 x 1/8000 degrees, or roughly 13.75 x 13.75 meters.
//
// See https://github.com/google/open-location-code.
package olc

import (
	"errors"
	"fmt"
	"math"
	"strings"
)

const (
	// Separator is the character that separates the two parts of location code.
	Separator = '+'
	// Padding is the optional (left) padding character.
	Padding = '0'

	// Alphabet is the set of valid encoding characters.
	Alphabet = "23456789CFGHJMPQRVWX"
	encBase  = len(Alphabet)

	maxCodeLen  = 15
	pairCodeLen = 10
	gridCodeLen = maxCodeLen - pairCodeLen
	gridCols    = 4
	gridRows    = 5
	// First place value of the pairs (if the last pair value is 1). encBase^(pairs-1)
	pairFPV = 160000
	// Precision of the pair part of the code, in 1/degrees.
	pairPrecision = 8000
	// Full value of the latitude grid - gridRows**gridCodeLen.
	gridLatFullValue = 3125
	// Full value of the longitude grid - gridCols**gridCodeLen.
	gridLngFullValue = 1024
	// First place value of the latitude grid (if the last place is 1). gridRows^(gridCodeLen - 1)
	gridLatFPV = gridLatFullValue / gridRows
	// First place value of the longitude grid (if the last place is 1). gridCols^(gridCodeLen - 1)
	gridLngFPV = gridLngFullValue / gridCols
	// Latitude precision of a full length code. pairPrecision * gridRows**gridCodeLen
	finalLatPrecision = pairPrecision * gridLatFullValue
	// Longitude precision of a full length code. pairPrecision * gridCols**gridCodeLen
	finalLngPrecision = pairPrecision * gridLngFullValue

	latMax = 90
	lngMax = 180

	sepPos = 8
)

// CodeArea is the area represented by a location code.
type CodeArea struct {
	LatLo, LngLo, LatHi, LngHi float64
	Len                        int
}

// Center returns the (lat,lng) of the center of the area.
func (area CodeArea) Center() (lat, lng float64) {
	return math.Min(area.LatLo+(area.LatHi-area.LatLo)/2, latMax),
		math.Min(area.LngLo+(area.LngHi-area.LngLo)/2, lngMax)
}

// Check checks whether the passed string is a valid OLC code.
// It could be a full code (8FVC9G8F+6W), a padded code (8FVC0000+) or a code fragment (9G8F+6W).
func Check(code string) error {
	if code == "" || len(code) == 1 && code[0] == Separator {
		return errors.New("empty code")
	}
	n := len(code)
	firstSep, firstPad := -1, -1
	for i, r := range code {
		if firstPad != -1 {
			// Open Location Codes with less than eight digits can be suffixed with zeros with a "+" used as the final character. Zeros may not be followed by any other digit.
			switch r {
			case Padding:
				continue
			case Separator:
				if firstSep != -1 {
					return fmt.Errorf("extraneous separator @%d", i)
				}
				firstSep = i
				if n-1 == i {
					continue
				}
			}
			return fmt.Errorf("%c after zero @%d", r, i)
		}

		if '2' <= r && r <= '9' {
			continue
		}
		switch r {
		case 'C', 'F', 'G', 'H', 'J', 'M', 'P', 'Q', 'R', 'V', 'W', 'X',
			// Processing of Open Location Codes must be case insensitive.
			'c', 'f', 'g', 'h', 'j', 'm', 'p', 'q', 'r', 'v', 'w', 'x':
			continue
		case Separator:
			// In addition to the above characters, a full Open Location Code can include a single "+" as a separator after the eighth digit.
			if firstSep != -1 {
				return fmt.Errorf("extra separator seen @%d", i)
			}
			if i > sepPos || i%2 == 1 {
				return fmt.Errorf("separator in illegal position @%d", i)
			}
			firstSep = i
		case Padding:
			if i == 0 {
				return errors.New("shouldn't start with padding character")
			}
			firstPad = i
		default:
			return fmt.Errorf("invalid char %c @%d", r, i)
		}
	}
	if firstSep == -1 {
		return errors.New("missing separator")
	}
	if n-firstSep-1 == 1 {
		return fmt.Errorf("only one char (%q) after separator", code[firstSep+1:])
	}
	if firstPad != -1 {
		if firstSep < sepPos {
			return errors.New("short codes cannot have padding")
		}
		if len(code)-firstPad-1%2 == 1 {
			return errors.New("odd number of padding chars")
		}
	}
	return nil
}

// CheckShort checks whether the passed string is a valid short code.
// If it is valid full code, then it returns ErrNotShort.
func CheckShort(code string) error {
	if err := Check(code); err != nil {
		return err
	}
	if i := strings.IndexByte(code, Separator); i >= 0 && i < sepPos {
		return nil
	}
	return ErrNotShort
}

// CheckFull checks whether the passed string is a valid full code.
// If it is short, it returns ErrShort.
func CheckFull(code string) error {
	if err := CheckShort(code); err == nil {
		return ErrShort
	} else if err != ErrNotShort {
		return err
	}
	if firstLat := strings.IndexByte(Alphabet, upper(code[0])) * encBase; firstLat >= latMax*2 {
		return errors.New("latitude outside range")
	}
	if len(code) == 1 {
		return nil
	}
	if firstLong := strings.IndexByte(Alphabet, upper(code[1])) * encBase; firstLong >= lngMax*2 {
		return errors.New("longitude outside range")
	}
	return nil
}

func upper(b byte) byte {
	if 'c' <= b && b <= 'x' {
		return b + 'C' - 'c'
	}
	return b
}

// StripCode strips the padding and separator characters from the code.
//
// The code is truncated to the first 15 digits, as Decode won't use more,
// to avoid underflow errors.
func StripCode(code string) string {
	code = strings.Map(
		func(r rune) rune {
			if r == Separator || r == Padding {
				return -1
			}
			return rune(upper(byte(r)))
		},
		code)
	if len(code) > maxCodeLen {
		return code[:maxCodeLen]
	}
	return code
}

// Because the OLC codes are an area, they can't start at 180 degrees, because they would then have something > 180 as their upper bound.
// Basically, what you have to do is normalize the longitude - so you need to change 180 degrees to -180 degrees.
func normalize(value, max float64) float64 {
	for value < -max {
		value += 2 * max
	}
	for value >= max {
		value -= 2 * max
	}
	return value
}

// clipLatitude forces the latitude into the valid range.
func clipLatitude(lat float64) float64 {
	if lat > latMax {
		return latMax
	}
	if lat < -latMax {
		return -latMax
	}
	return lat
}

func normalizeLat(value float64) float64 {
	return normalize(value, latMax)
}

func normalizeLng(value float64) float64 {
	return normalize(value, lngMax)
}