crosspoint-reader/lib/Epub/Epub/ParsedText.cpp

#include "ParsedText.h"

#include <GfxRenderer.h>

#include <algorithm>
#include <cmath>
#include <functional>
#include <limits>
#include <vector>

constexpr int MAX_COST = std::numeric_limits<int>::max();

void ParsedText::addWord(std::string word, const EpdFontStyle fontStyle) {
  if (word.empty()) return;

  words.push_back(std::move(word));
  wordStyles.push_back(fontStyle);
}

// Consumes data to minimize memory usage
void ParsedText::layoutAndExtractLines(const GfxRenderer& renderer, const int fontId, const int horizontalMargin,
                                       const std::function<void(std::shared_ptr<TextBlock>)>& processLine,
                                       const bool includeLastLine) {
  if (words.empty()) {
    return;
  }

  const int pageWidth = renderer.getScreenWidth() - horizontalMargin;
  const int spaceWidth = renderer.getSpaceWidth(fontId);
  const auto wordWidths = calculateWordWidths(renderer, fontId);
  const auto lineBreakIndices = computeLineBreaks(pageWidth, spaceWidth, wordWidths);
  const size_t lineCount = includeLastLine ? lineBreakIndices.size() : lineBreakIndices.size() - 1;

  for (size_t i = 0; i < lineCount; ++i) {
    extractLine(i, pageWidth, spaceWidth, wordWidths, lineBreakIndices, processLine);
  }
}

std::vector<uint16_t> ParsedText::calculateWordWidths(const GfxRenderer& renderer, const int fontId) {
  const size_t totalWordCount = words.size();

  std::vector<uint16_t> wordWidths;
  wordWidths.reserve(totalWordCount);

  // add em-space at the beginning of first word in paragraph to indent
  if (!extraParagraphSpacing) {
    std::string& first_word = words.front();
    first_word.insert(0, "\xe2\x80\x83");
  }

  auto wordsIt = words.begin();
  auto wordStylesIt = wordStyles.begin();

  while (wordsIt != words.end()) {
    wordWidths.push_back(renderer.getTextWidth(fontId, wordsIt->c_str(), *wordStylesIt));

    std::advance(wordsIt, 1);
    std::advance(wordStylesIt, 1);
  }

  return wordWidths;
}

std::vector<size_t> ParsedText::computeLineBreaks(const int pageWidth, const int spaceWidth,
                                                  const std::vector<uint16_t>& wordWidths) const {
  const size_t totalWordCount = words.size();

  // DP table to store the minimum badness (cost) of lines starting at index i
  std::vector<int> dp(totalWordCount);
  // 'ans[i]' stores the index 'j' of the *last word* in the optimal line starting at 'i'
  std::vector<size_t> ans(totalWordCount);

  // Base Case
  dp[totalWordCount - 1] = 0;
  ans[totalWordCount - 1] = totalWordCount - 1;

  for (int i = totalWordCount - 2; i >= 0; --i) {
    int currlen = -spaceWidth;
    dp[i] = MAX_COST;

    for (size_t j = i; j < totalWordCount; ++j) {
      // Current line length: previous width + space + current word width
      currlen += wordWidths[j] + spaceWidth;

      if (currlen > pageWidth) {
        break;
      }

      int cost;
      if (j == totalWordCount - 1) {
        cost = 0;  // Last line
      } else {
        const int remainingSpace = pageWidth - currlen;
        // Use long long for the square to prevent overflow
        const long long cost_ll = static_cast<long long>(remainingSpace) * remainingSpace + dp[j + 1];

        if (cost_ll > MAX_COST) {
          cost = MAX_COST;
        } else {
          cost = static_cast<int>(cost_ll);
        }
      }

      if (cost < dp[i]) {
        dp[i] = cost;
        ans[i] = j;  // j is the index of the last word in this optimal line
      }
    }
  }

  // Stores the index of the word that starts the next line (last_word_index + 1)
  std::vector<size_t> lineBreakIndices;
  size_t currentWordIndex = 0;

  while (currentWordIndex < totalWordCount) {
    size_t nextBreakIndex = ans[currentWordIndex] + 1;

    // Safety check: prevent infinite loop if nextBreakIndex doesn't advance
    if (nextBreakIndex <= currentWordIndex) {
      // Force advance by at least one word to avoid infinite loop
      nextBreakIndex = currentWordIndex + 1;
    }

    lineBreakIndices.push_back(nextBreakIndex);
    currentWordIndex = nextBreakIndex;
  }

  return lineBreakIndices;
}

void ParsedText::extractLine(const size_t breakIndex, const int pageWidth, const int spaceWidth,
                             const std::vector<uint16_t>& wordWidths, const std::vector<size_t>& lineBreakIndices,
                             const std::function<void(std::shared_ptr<TextBlock>)>& processLine) {
  const size_t lineBreak = lineBreakIndices[breakIndex];
  const size_t lastBreakAt = breakIndex > 0 ? lineBreakIndices[breakIndex - 1] : 0;
  const size_t lineWordCount = lineBreak - lastBreakAt;

  // Calculate total word width for this line
  int lineWordWidthSum = 0;
  for (size_t i = lastBreakAt; i < lineBreak; i++) {
    lineWordWidthSum += wordWidths[i];
  }

  // Calculate spacing
  const int spareSpace = pageWidth - lineWordWidthSum;

  int spacing = spaceWidth;
  const bool isLastLine = breakIndex == lineBreakIndices.size() - 1;

  if (style == TextBlock::JUSTIFIED && !isLastLine && lineWordCount >= 2) {
    spacing = spareSpace / (lineWordCount - 1);
  }

  // Calculate initial x position
  uint16_t xpos = 0;
  if (style == TextBlock::RIGHT_ALIGN) {
    xpos = spareSpace - (lineWordCount - 1) * spaceWidth;
  } else if (style == TextBlock::CENTER_ALIGN) {
    xpos = (spareSpace - (lineWordCount - 1) * spaceWidth) / 2;
  }

  // Pre-calculate X positions for words
  std::list<uint16_t> lineXPos;
  for (size_t i = lastBreakAt; i < lineBreak; i++) {
    const uint16_t currentWordWidth = wordWidths[i];
    lineXPos.push_back(xpos);
    xpos += currentWordWidth + spacing;
  }

  // Iterators always start at the beginning as we are moving content with splice below
  auto wordEndIt = words.begin();
  auto wordStyleEndIt = wordStyles.begin();
  std::advance(wordEndIt, lineWordCount);
  std::advance(wordStyleEndIt, lineWordCount);

  // *** CRITICAL STEP: CONSUME DATA USING SPLICE ***
  std::list<std::string> lineWords;
  lineWords.splice(lineWords.begin(), words, words.begin(), wordEndIt);
  std::list<EpdFontStyle> lineWordStyles;
  lineWordStyles.splice(lineWordStyles.begin(), wordStyles, wordStyles.begin(), wordStyleEndIt);

  processLine(std::make_shared<TextBlock>(std::move(lineWords), std::move(lineXPos), std::move(lineWordStyles), style));
}