using System.Globalization;

namespace WxAgent.Core;

public static class WechatTextInput
{
    public const int MaxSegmentLength = 4000;
    public const int MaxTextLength = 20_000;

    public static string Prepare(string text)
    {
        var normalized = PrepareLongText(text);
        if (normalized.Length > MaxSegmentLength)
            throw new WxAgentException(WxAgentErrorCode.InvalidArgument, $"Message text must not exceed {MaxSegmentLength} characters.");
        return normalized;
    }

    public static string PrepareLongText(string text)
    {
        if (string.IsNullOrWhiteSpace(text))
            throw new WxAgentException(WxAgentErrorCode.InvalidArgument, "Message text must not be empty.");
        var normalized = text.ReplaceLineEndings("\n");
        if (normalized.Length > MaxTextLength || normalized.Any(character => char.IsControl(character) && character is not ('\n' or '\t')))
            throw new WxAgentException(WxAgentErrorCode.InvalidArgument, $"Message text must not exceed {MaxTextLength} characters or contain unsupported control characters.");
        return normalized;
    }

    public static IReadOnlyList<string> SplitForSend(string text)
    {
        var normalized = PrepareLongText(text);
        if (normalized.Length <= MaxSegmentLength)
            return [normalized];

        var textElementStarts = StringInfo.ParseCombiningCharacters(normalized);
        var segments = new List<string>();
        var start = 0;
        while (start < normalized.Length)
        {
            var end = Math.Min(start + MaxSegmentLength, normalized.Length);
            if (end < normalized.Length)
            {
                var boundaryIndex = Array.BinarySearch(textElementStarts, end);
                if (boundaryIndex < 0) boundaryIndex = ~boundaryIndex - 1;
                if (boundaryIndex < 0 || textElementStarts[boundaryIndex] <= start)
                    throw new WxAgentException(WxAgentErrorCode.InvalidArgument, "A text element exceeds the per-message length limit.");
                end = textElementStarts[boundaryIndex];
            }

            segments.Add(normalized[start..end]);
            start = end;
        }
        return segments;
    }
}