package com.lemenzo.calculator.engine

import java.math.BigDecimal
import java.math.MathContext
import java.math.RoundingMode

object DisplayFormatter {
    enum class Notation { AUTO, SCIENTIFIC, ENGINEERING }
    private val mc = MathContext(34, RoundingMode.HALF_EVEN)

    fun format(
        value: BigDecimal,
        notation: Notation = Notation.AUTO,
        decimalSeparator: Char = '.',
        groupingSeparator: Char = ',',
        grouping: Boolean = true
    ): String {
        if (value.compareTo(BigDecimal.ZERO) == 0) return "0"
        val rounded = value.round(mc).stripTrailingZeros()
        val raw = when (notation) {
            Notation.SCIENTIFIC -> scientific(rounded)
            Notation.ENGINEERING -> rounded.toEngineeringString()
            Notation.AUTO -> {
                val a = rounded.abs()
                if (a >= BigDecimal("1E15") || a < BigDecimal("1E-10")) rounded.toEngineeringString()
                else rounded.toPlainString()
            }
        }
        return localize(raw.replace("E+", "E"), decimalSeparator, groupingSeparator, grouping && !raw.contains('E'))
    }

    private fun scientific(v: BigDecimal): String {
        val x = v.stripTrailingZeros()
        val exponent = x.precision() - 1 - x.scale()
        val mantissa = x.movePointLeft(exponent).stripTrailingZeros().toPlainString()
        return "$mantissa" + "E" + exponent
    }

    private fun localize(raw: String, decimal: Char, grouping: Char, group: Boolean): String {
        val e = raw.indexOf('E')
        val number = if (e >= 0) raw.substring(0, e) else raw
        val exponent = if (e >= 0) raw.substring(e) else ""
        val sign = if (number.startsWith("-")) "-" else ""
        val unsigned = number.removePrefix("-")
        val dot = unsigned.indexOf('.')
        val integer = if (dot >= 0) unsigned.substring(0, dot) else unsigned
        val fraction = if (dot >= 0) unsigned.substring(dot + 1) else ""
        val grouped = if (group) groupDigits(integer, grouping) else integer
        val localized = if (fraction.isEmpty()) "$sign$grouped" else "$sign$grouped$decimal$fraction"
        return localized + exponent.replace('.', decimal)
    }

    private fun groupDigits(value: String, separator: Char): String {
        if (value.length <= 3) return value
        val first = value.length % 3
        val out = StringBuilder()
        var index = 0
        if (first != 0) {
            out.append(value.substring(0, first))
            index = first
        }
        while (index < value.length) {
            if (out.isNotEmpty()) out.append(separator)
            out.append(value.substring(index, index + 3))
            index += 3
        }
        return out.toString()
    }
}