package channels

import (
	"strings"
)

// SplitMessage splits long messages into chunks, preserving code block integrity.
// The maxLen parameter is measured in runes (Unicode characters), not bytes.
// The function reserves a buffer (10% of maxLen, min 50) to leave room for closing code blocks,
// but may extend to maxLen when needed.
// Call SplitMessage with the full text content and the maximum allowed length of a single message;
// it returns a slice of message chunks that each respect maxLen and avoid splitting fenced code blocks.
func SplitMessage(content string, maxLen int) []string {
	if maxLen <= 0 {
		if content == "" {
			return nil
		}
		return []string{content}
	}

	runes := []rune(content)
	totalLen := len(runes)
	var messages []string

	// Dynamic buffer: 10% of maxLen, but at least 50 chars if possible
	codeBlockBuffer := maxLen / 10
	if codeBlockBuffer < 50 {
		codeBlockBuffer = 50
	}
	if codeBlockBuffer > maxLen/2 {
		codeBlockBuffer = maxLen / 2
	}

	start := 0
	for start < totalLen {
		remaining := totalLen - start
		if remaining <= maxLen {
			messages = append(messages, string(runes[start:totalLen]))
			break
		}

		// Effective split point: maxLen minus buffer, to leave room for code blocks
		effectiveLimit := maxLen - codeBlockBuffer
		if effectiveLimit < maxLen/2 {
			effectiveLimit = maxLen / 2
		}

		end := start + effectiveLimit

		// Find natural split point within the effective limit
		msgEnd := findLastNewlineInRange(runes, start, end, 200)
		if msgEnd <= start {
			msgEnd = findLastSpaceInRange(runes, start, end, 100)
		}
		if msgEnd <= start {
			msgEnd = end
		}

		// Check if this would end with an incomplete code block
		unclosedIdx := findLastUnclosedCodeBlockInRange(runes, start, msgEnd)

		if unclosedIdx >= 0 {
			// Message would end with incomplete code block
			// Try to extend up to maxLen to include the closing ```
			if totalLen > msgEnd {
				closingIdx := findNextClosingCodeBlockInRange(runes, msgEnd, totalLen)
				if closingIdx > 0 && closingIdx-start <= maxLen {
					// Extend to include the closing ```
					msgEnd = closingIdx
				} else {
					// Code block is too long to fit in one chunk or missing closing fence.
					// Try to split inside by injecting closing and reopening fences.
					headerEnd := findNewlineFrom(runes, unclosedIdx)
					var header string
					if headerEnd == -1 {
						header = strings.TrimSpace(string(runes[unclosedIdx : unclosedIdx+3]))
					} else {
						header = strings.TrimSpace(string(runes[unclosedIdx:headerEnd]))
					}
					headerEndIdx := unclosedIdx + len([]rune(header))
					if headerEnd != -1 {
						headerEndIdx = headerEnd
					}

					// If we have a reasonable amount of content after the header, split inside
					if msgEnd > headerEndIdx+20 {
						// Find a better split point closer to maxLen
						innerLimit := start + maxLen - 5 // Leave room for "\n```"
						if innerLimit > totalLen {
							innerLimit = totalLen
						}
						betterEnd := findLastNewlineInRange(runes, start, innerLimit, 200)
						if betterEnd > headerEndIdx {
							msgEnd = betterEnd
						} else {
							msgEnd = innerLimit
						}
						chunk := strings.TrimRight(string(runes[start:msgEnd]), " \t\n\r") + "\n```"
						messages = append(messages, chunk)
						remaining := strings.TrimSpace(header + "\n" + string(runes[msgEnd:totalLen]))
						// Replace the tail of runes with the reconstructed remaining
						runes = []rune(remaining)
						totalLen = len(runes)
						start = 0
						continue
					}

					// Otherwise, try to split before the code block starts
					newEnd := findLastNewlineInRange(runes, start, unclosedIdx, 200)
					if newEnd <= start {
						newEnd = findLastSpaceInRange(runes, start, unclosedIdx, 100)
					}
					if newEnd > start {
						msgEnd = newEnd
					} else {
						// If we can't split before, we MUST split inside (last resort)
						if unclosedIdx-start > 20 {
							msgEnd = unclosedIdx
						} else {
							splitAt := start + maxLen - 5
							if splitAt > totalLen {
								splitAt = totalLen
							}
							chunk := strings.TrimRight(string(runes[start:splitAt]), " \t\n\r") + "\n```"
							messages = append(messages, chunk)
							remaining := strings.TrimSpace(header + "\n" + string(runes[splitAt:totalLen]))
							runes = []rune(remaining)
							totalLen = len(runes)
							start = 0
							continue
						}
					}
				}
			}
		}

		if msgEnd <= start {
			msgEnd = start + effectiveLimit
		}

		messages = append(messages, string(runes[start:msgEnd]))
		// Advance start, skipping leading whitespace of next chunk
		start = msgEnd
		for start < totalLen && (runes[start] == ' ' || runes[start] == '\t' || runes[start] == '\n' || runes[start] == '\r') {
			start++
		}
	}

	return messages
}

// findLastUnclosedCodeBlockInRange finds the last opening ``` that doesn't have a closing ```
// within runes[start:end]. Returns the absolute rune index or -1.
func findLastUnclosedCodeBlockInRange(runes []rune, start, end int) int {
	inCodeBlock := false
	lastOpenIdx := -1

	for i := start; i < end; i++ {
		if i+2 < end && runes[i] == '`' && runes[i+1] == '`' && runes[i+2] == '`' {
			if !inCodeBlock {
				lastOpenIdx = i
			}
			inCodeBlock = !inCodeBlock
			i += 2
		}
	}

	if inCodeBlock {
		return lastOpenIdx
	}
	return -1
}

// findNextClosingCodeBlockInRange finds the next closing ``` starting from startIdx
// within runes[startIdx:end]. Returns the absolute index after the closing ``` or -1.
func findNextClosingCodeBlockInRange(runes []rune, startIdx, end int) int {
	for i := startIdx; i < end; i++ {
		if i+2 < end && runes[i] == '`' && runes[i+1] == '`' && runes[i+2] == '`' {
			return i + 3
		}
	}
	return -1
}

// findNewlineFrom finds the first newline character starting from the given index.
// Returns the absolute index or -1 if not found.
func findNewlineFrom(runes []rune, from int) int {
	for i := from; i < len(runes); i++ {
		if runes[i] == '\n' {
			return i
		}
	}
	return -1
}

// findLastNewlineInRange finds the last newline within the last searchWindow runes
// of the range runes[start:end]. Returns the absolute index or start-1 (indicating not found).
func findLastNewlineInRange(runes []rune, start, end, searchWindow int) int {
	searchStart := end - searchWindow
	if searchStart < start {
		searchStart = start
	}
	for i := end - 1; i >= searchStart; i-- {
		if runes[i] == '\n' {
			return i
		}
	}
	return start - 1
}

// findLastSpaceInRange finds the last space/tab within the last searchWindow runes
// of the range runes[start:end]. Returns the absolute index or start-1 (indicating not found).
func findLastSpaceInRange(runes []rune, start, end, searchWindow int) int {
	searchStart := end - searchWindow
	if searchStart < start {
		searchStart = start
	}
	for i := end - 1; i >= searchStart; i-- {
		if runes[i] == ' ' || runes[i] == '\t' {
			return i
		}
	}
	return start - 1
}