#nullable enable

using System.Globalization;

using System.Text;

using System.Text.RegularExpressions;

using ExcelDataReader.Misc;

namespace ExcelDataReader.Core;

/// <summary>

/// Helpers class.

/// </summary>

internal static partial class Helpers

{

#if !NET8_0_OR_GREATER

private static readonly Regex EscapeRegexInstance = new("_x([0-9A-F]{4,4})_", RegexOptions.Compiled);

#endif

private static readonly char[] SingleByteEncodingHelper = ['a'];

/// <summary>

/// Determines whether the encoding is single byte or not.

/// </summary>

/// <param name="encoding">The encoding.</param>

/// <returns>

/// <see langword="true"/> if the specified encoding is single byte; otherwise, <see langword="false"/>.

/// </returns>

public static bool IsSingleByteEncoding(Encoding encoding)

{

return encoding.GetByteCount(SingleByteEncodingHelper) == 1;

}

public static string ConvertEscapeChars(string input)

{

// Fast rejection: the escape pattern always starts with "_x". For typical

// spreadsheet strings this short-circuits before the regex engine is entered.

if (input.IndexOf("_x", StringComparison.Ordinal) < 0)

return input;

return EscapeRegex().Replace(input, m => ((char)uint.Parse(m.Groups[1].Value, NumberStyles.HexNumber, CultureInfo.InvariantCulture)).ToString());

}

public static object ConvertFromOATime(double value, bool date1904)

{

var dateValue = AdjustOADateTime(value, date1904);

if (IsValidOADateTime(dateValue))

return DateTimeHelper.FromOADate(dateValue);

return value;

}

public static object ConvertFromOATime(int value, bool date1904)

{

var dateValue = AdjustOADateTime(value, date1904);

if (IsValidOADateTime(dateValue))

return DateTimeHelper.FromOADate(dateValue);

return value;

}

public static bool StringStartsWith(string value, char start)

{

#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER

return value.StartsWith(start);

#else

return value.Length > 0 && value[0] == start;

#endif

}

/// <summary>

/// Convert a double from Excel to an OA DateTime double.

/// The returned value is normalized to the '1900' date mode and adjusted for the 1900 leap year bug.

/// </summary>

private static double AdjustOADateTime(double value, bool date1904)

{

if (!date1904)

{

// Workaround for 1900 leap year bug in Excel

if (value is >= 0.0 and < 60.0)

{

return value + 1;

}

}

else

{

return value + 1462.0;

}

return value;

}

private static bool IsValidOADateTime(double value)

{

return value is > DateTimeHelper.OADateMinAsDouble and < DateTimeHelper.OADateMaxAsDouble;

}

#if NET8_0_OR_GREATER

[GeneratedRegex("_x([0-9A-F]{4,4})_")]

private static partial Regex EscapeRegex();

#else

private static Regex EscapeRegex() => EscapeRegexInstance;

#endif

}