package main

func isNumber(s string) bool {
	// Indices of number begin, dot, exp, number end (not included)
	begIdx, dotIdx, expIdx, endIdx := -1, -1, -1, -1
	// Has significant digit, has exp digit, previous one is digit
	sigDigit, expDigit, lDigit := false, true, false
	for i := range s {
		switch s[i] {
		case ' ':
			if begIdx != -1 {
				endIdx = i
			}
			lDigit = false
		case '+', '-':
			if begIdx == -1 {
				begIdx = i
			} else if i != expIdx+1 { // If not at begin and not after exp
				return false
			}
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			if !lDigit { // Is the first digit
				if endIdx != -1 { // Appears after end
					return false
				}
				if begIdx == -1 {
					begIdx = i
				}
				lDigit = true
				if expIdx == -1 { // Update sig digit & exp digit
					sigDigit = true
				} else {
					expDigit = true
				}
			}
		case '.':
			// If: has two dots || appears after exp || appears after end
			if dotIdx != -1 || expIdx != -1 || (!lDigit && endIdx != -1) {
				return false
			}
			if begIdx == -1 {
				begIdx = i
			}
			dotIdx = i
		case 'e':
			// If: has two exp || no digit at left side || appears after end
			if expIdx != -1 || !lDigit || endIdx != -1 {
				return false
			}
			expIdx = i
			lDigit = false
			expDigit = false // Needs exp digit
		default:
			return false
		}
	}
	return sigDigit && expDigit
}

// func main() {
// 	println(isNumber("   23 "))
// 	println(isNumber("6e-1"))
// 	println(isNumber("-53.5e+93"))
// 	println(isNumber(". 1"))
// 	println(isNumber("0e-"))
// 	println(isNumber("e-5"))
// 	println(isNumber(""))
// 	println(isNumber("1 a"))
// 	println(isNumber("--1"))
// 	println(isNumber("1. 31"))
// 	println(isNumber(".e-4"))
// 	println(isNumber("-."))
// }