kleingrep/regex/range2regex.go

package regex

import (
	"fmt"
	"math"
	"slices"
	"strconv"
	"strings"
)

type numRange struct {
	start int
	end   int
}

// Returns the exponent of the closest power of 10 smaller
// than the given value.
func floorPower10(val int) int {
	return int(math.Floor(math.Log10(float64(val))))
}

// Returns smallest multiple of 10^exp, that is greater than val
func roundUpToNearest10Multiple(val int, exp int) int {
	bench := int(math.Round(math.Pow10(exp)))
	if val != 0 && val%bench == 0 {
		return val
	} else {
		return (bench - val%bench) + val
	}
}

func roundDownToNearest10Multiple(val int, exp int) int {
	bench := int(math.Round(math.Pow10(exp)))
	return val - val%bench
}

// Converts the given integer into an int-slice, where each element
// represents a digit of the number.
func intToSlc(val int) []int {
	valStr := strconv.Itoa(val)
	valSlc := []rune(valStr)
	toRet := make([]int, len(valStr))
	for i, r := range valSlc {
		toRet[i] = int(r - 48)
	}
	return toRet
}

func range2regex(start int, end int) (string, error) {
	rangeStart := start
	rangeEnd := end
	if rangeStart > rangeEnd {
		return "", fmt.Errorf("numeric range start greater than range end")
	}

	ranges := make([]numRange, 0)
	// If both numbers are in the same power of 10 eg. 15000 and 17000.
	// the maximum power of 10 that we will go to, is determined by the largest
	// power of 10 at which both numbers differ. Given 15000 and 17000, we will
	// go up to 10^3, because that is the largestindex at which they differ.
	startRangeSlc := intToSlc(rangeStart)
	endRangeSlc := intToSlc(rangeEnd)
	maxPower10 := 0
	if len(startRangeSlc) != len(endRangeSlc) { // Different number of digits, so we will go up to the maximum (which must be rangeEnd)
		maxPower10 = floorPower10(rangeEnd) // Maximum power of 10 that we will reach
	} else {
		maxPower10 = 0
		for i := range startRangeSlc {
			if startRangeSlc[i] != endRangeSlc[i] {
				maxPower10 = len(startRangeSlc) - i - 1
				break
			}
		}
	}

	tmp := rangeStart
	exp := 1 // The exponent of 10 that we are finding the range to

	// Increasing up to highest power
	for exp <= maxPower10 {
		tmpRangeEnd := roundUpToNearest10Multiple(tmp, exp)
		if tmp != tmpRangeEnd {
			ranges = append(ranges, numRange{tmp, tmpRangeEnd - 1})
		}
		tmp = tmpRangeEnd
		exp++
	}

	exp--

	// Decreasing down to lowest power
	for exp >= 1 {
		tmpRangeEnd := roundDownToNearest10Multiple(rangeEnd, exp)
		if tmp != tmpRangeEnd {
			ranges = append(ranges, numRange{tmp, tmpRangeEnd - 1})
		}
		tmp = tmpRangeEnd
		exp--
	}

	// Last range - tmp to rangeEnd
	ranges = append(ranges, numRange{tmp, rangeEnd})

	regexSlice := make([]string, 0)
	// Generate the regex
	for _, rg := range ranges {
		tmpStr := ""
		tmpStr += string(nonCapLparenRune)
		startSlc := intToSlc(rg.start)
		endSlc := intToSlc(rg.end)
		if len(startSlc) != len(endSlc) {
			return "", fmt.Errorf("error parsing numeric range")
		}
		for i := range startSlc {
			if startSlc[i] == endSlc[i] {
				tmpStr += string(rune(startSlc[i] + 48)) // '0' is ascii value 48, 1 is 49 etc. To convert the digit to its character form, we can just add 48.
			} else {
				tmpStr += fmt.Sprintf("%c%c-%c%c", lbracketRune, rune(startSlc[i]+48), rune(endSlc[i]+48), rbracketRune)
			}
		}
		tmpStr += ")"
		regexSlice = append(regexSlice, tmpStr)
	}
	// Each element of the slice represents one 'group'. Taking 0-255 as an example, the elements would be:
	// 	1. 0-9
	// 	2. 10-99
	// 	3. 100-199
	// 	4. 200-249
	// 	5. 250-255
	//
	// The reason this is reversed before joining it, is because it is incompatible with the PCRE rule for matching.
	// The PCRE rule specifies that the left-branch of an alternation is preferred. Even though this engine uses the POSIX
	// rule at the moment (which prefers the longest match regardless of the order of the alternation), reversing the string
	// has no downsides. It doesn't affect POSIX matching, and it will reduce my burden if I decide to switch to PCRE matching.
	slices.Reverse(regexSlice)
	regex := string(nonCapLparenRune) + strings.Join(regexSlice, "|") + ")"
	return regex, nil

}