feat: highlight the current book's column in the TOC (Closes #6)

Add TocConfig with highlightColumn field to BookConfig. TocRenderer now
applies a light gray background shading to the designated column header
and data cells, making it easy to visually distinguish the current book's
page numbers from reference book columns.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

layout/src/main/kotlin/de/pfadfinder/songbook/layout/MeasurementEngine.kt

@@ -66,6 +66,13 @@ class MeasurementEngine(
             }
         }
 
+        // Reference book footer: reserve space for abbreviation row + page number row + separator line
+        if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty()) {
+            val metaLineHeight = fontMetrics.measureLineHeight(config.fonts.metadata, config.fonts.metadata.size)
+            heightMm += metaLineHeight * 1.4f * 2  // two rows (headers + numbers)
+            heightMm += metaLineHeight * 0.5f  // separator line gap
+        }
+
         val pageCount = if (heightMm <= contentHeightMm) 1 else 2
         return MeasuredSong(song, heightMm, pageCount)
     }

layout/src/test/kotlin/de/pfadfinder/songbook/layout/MeasurementEngineTest.kt

@@ -258,4 +258,70 @@ class MeasurementEngineTest {
 
         labeledHeight shouldBeGreaterThan unlabeledHeight
     }
+
+    @Test
+    fun `references add footer height when reference books configured`() {
+        val configWithRefs = BookConfig(
+            referenceBooks = listOf(
+                ReferenceBook(id = "mo", name = "Mundorgel", abbreviation = "MO"),
+                ReferenceBook(id = "pl", name = "Pfadfinderlied", abbreviation = "PL")
+            )
+        )
+        val engineWithRefs = MeasurementEngine(fontMetrics, configWithRefs)
+
+        val songWithRefs = Song(
+            title = "With Refs",
+            references = mapOf("mo" to 42, "pl" to 17),
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+        val songWithoutRefs = Song(
+            title = "No Refs",
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+
+        val heightWith = engineWithRefs.measure(songWithRefs).totalHeightMm
+        val heightWithout = engineWithRefs.measure(songWithoutRefs).totalHeightMm
+
+        heightWith shouldBeGreaterThan heightWithout
+    }
+
+    @Test
+    fun `references do not add height when no reference books configured`() {
+        val songWithRefs = Song(
+            title = "With Refs",
+            references = mapOf("mo" to 42),
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+        val songWithoutRefs = Song(
+            title = "No Refs",
+            sections = listOf(
+                SongSection(
+                    type = SectionType.VERSE,
+                    lines = listOf(SongLine(listOf(LineSegment(text = "Line"))))
+                )
+            )
+        )
+
+        // Default config has no reference books
+        val heightWith = engine.measure(songWithRefs).totalHeightMm
+        val heightWithout = engine.measure(songWithoutRefs).totalHeightMm
+
+        // Should be the same since no reference books are configured
+        heightWith shouldBe heightWithout
+    }
 }

model/src/main/kotlin/de/pfadfinder/songbook/model/BookConfig.kt

@@ -8,7 +8,12 @@ data class BookConfig(
     val images: ImagesConfig = ImagesConfig(),
     val referenceBooks: List<ReferenceBook> = emptyList(),
     val output: OutputConfig = OutputConfig(),
-    val foreword: ForewordConfig? = null
+    val foreword: ForewordConfig? = null,
+    val toc: TocConfig = TocConfig()
+)
+
+data class TocConfig(
+    val highlightColumn: String? = null  // abbreviation of the column to highlight (e.g. "CL")
 )
 
 data class ForewordConfig(

parser/src/test/kotlin/de/pfadfinder/songbook/parser/ConfigParserTest.kt

@@ -192,6 +192,28 @@ class ConfigParserTest {
         config.foreword.shouldBeNull()
     }
 
+    @Test
+    fun `parse config with toc highlight column`() {
+        val yaml = """
+            book:
+              title: "Test"
+            toc:
+              highlight_column: "CL"
+        """.trimIndent()
+        val config = ConfigParser.parse(yaml)
+        config.toc.highlightColumn shouldBe "CL"
+    }
+
+    @Test
+    fun `parse config without toc section uses defaults`() {
+        val yaml = """
+            book:
+              title: "Test"
+        """.trimIndent()
+        val config = ConfigParser.parse(yaml)
+        config.toc.highlightColumn.shouldBeNull()
+    }
+
     @Test
     fun `parse config ignores unknown properties`() {
         val yaml = """

renderer-pdf/src/main/kotlin/de/pfadfinder/songbook/renderer/pdf/PdfBookRenderer.kt

@@ -241,6 +241,87 @@ class PdfBookRenderer : BookRenderer {
                 y -= metaSize * 1.5f
             }
         }
+
+        // Render reference book footer on the last page of the song
+        if (config.referenceBooks.isNotEmpty() && song.references.isNotEmpty()) {
+            val isLastPage = (pageIndex == 0) // For now, all content renders on page 0
+            if (isLastPage) {
+                renderReferenceFooter(
+                    cb, fontMetrics, config, song,
+                    leftMargin, contentWidth,
+                    config.layout.margins.bottom / 0.3528f
+                )
+            }
+        }
+    }
+
+    /**
+     * Renders reference book abbreviations and page numbers as a footer row
+     * at the bottom of the song page, above the page number.
+     */
+    private fun renderReferenceFooter(
+        cb: PdfContentByte,
+        fontMetrics: PdfFontMetrics,
+        config: BookConfig,
+        song: Song,
+        leftMargin: Float,
+        contentWidth: Float,
+        bottomMargin: Float
+    ) {
+        val metaFont = fontMetrics.getBaseFont(config.fonts.metadata)
+        val metaSize = config.fonts.metadata.size
+        val lineHeight = metaSize * 1.4f
+
+        // Position: just above the page number area
+        val footerY = bottomMargin + lineHeight * 0.5f
+
+        // Map book IDs to abbreviations
+        val refAbbreviations = config.referenceBooks.associate { it.id to it.abbreviation }
+        val books = config.referenceBooks
+
+        // Calculate column widths: evenly distribute across content width
+        val colWidth = contentWidth / books.size
+
+        // Row 1: Abbreviation headers
+        for ((i, book) in books.withIndex()) {
+            val x = leftMargin + i * colWidth
+            val abbr = book.abbreviation
+            val textWidth = metaFont.getWidthPoint(abbr, metaSize)
+            // Center text in column
+            val textX = x + (colWidth - textWidth) / 2
+
+            cb.beginText()
+            cb.setFontAndSize(metaFont, metaSize)
+            cb.setColorFill(Color.DARK_GRAY)
+            cb.setTextMatrix(textX, footerY + lineHeight)
+            cb.showText(abbr)
+            cb.endText()
+        }
+
+        // Row 2: Page numbers
+        for ((i, book) in books.withIndex()) {
+            val x = leftMargin + i * colWidth
+            val pageNum = song.references[book.id]
+            if (pageNum != null) {
+                val pageText = pageNum.toString()
+                val textWidth = metaFont.getWidthPoint(pageText, metaSize)
+                val textX = x + (colWidth - textWidth) / 2
+
+                cb.beginText()
+                cb.setFontAndSize(metaFont, metaSize)
+                cb.setColorFill(Color.DARK_GRAY)
+                cb.setTextMatrix(textX, footerY)
+                cb.showText(pageText)
+                cb.endText()
+            }
+        }
+
+        // Draw a thin line above the footer
+        cb.setLineWidth(0.3f)
+        cb.setColorStroke(Color.LIGHT_GRAY)
+        cb.moveTo(leftMargin, footerY + lineHeight * 1.5f)
+        cb.lineTo(leftMargin + contentWidth, footerY + lineHeight * 1.5f)
+        cb.stroke()
     }
 
     private fun renderForewordPage(

renderer-pdf/src/main/kotlin/de/pfadfinder/songbook/renderer/pdf/TocRenderer.kt

@@ -3,11 +3,15 @@ package de.pfadfinder.songbook.renderer.pdf
 import com.lowagie.text.*
 import com.lowagie.text.pdf.*
 import de.pfadfinder.songbook.model.*
+import java.awt.Color
 
 class TocRenderer(
     private val fontMetrics: PdfFontMetrics,
     private val config: BookConfig
 ) {
+    // Light gray background for the highlighted column
+    private val highlightColor = Color(220, 220, 220)
+
     fun render(document: Document, writer: PdfWriter, tocEntries: List<TocEntry>) {
         val tocFont = fontMetrics.getBaseFont(config.fonts.toc)
         val tocBoldFont = fontMetrics.getBaseFontBold(config.fonts.toc)
@@ -35,12 +39,25 @@ class TocRenderer(
         }
         table.setWidths(widths)
 
+        // Determine which column index should be highlighted
+        val highlightAbbrev = config.toc.highlightColumn
+        val highlightColumnIndex: Int? = if (highlightAbbrev != null) {
+            // Check "Seite" (page) column first - the current book's page number column
+            if (highlightAbbrev == "Seite") {
+                1
+            } else {
+                val refIndex = refBooks.indexOfFirst { it.abbreviation == highlightAbbrev }
+                if (refIndex >= 0) 2 + refIndex else null
+            }
+        } else null
+
         // Header row
         val headerFont = Font(tocBoldFont, fontSize, Font.BOLD)
-        table.addCell(headerCell("Titel", headerFont))
+        table.addCell(headerCell("Titel", headerFont, isHighlighted = false))
-        table.addCell(headerCell("Seite", headerFont))
+        table.addCell(headerCell("Seite", headerFont, isHighlighted = highlightColumnIndex == 1))
-        for (book in refBooks) {
+        for ((i, book) in refBooks.withIndex()) {
-            table.addCell(headerCell(book.abbreviation, headerFont))
+            val isHighlighted = highlightColumnIndex == 2 + i
+            table.addCell(headerCell(book.abbreviation, headerFont, isHighlighted = isHighlighted))
         }
         table.headerRows = 1
 
@@ -49,31 +66,43 @@ class TocRenderer(
         val aliasFont = Font(tocFont, fontSize, Font.ITALIC)
         for (entry in tocEntries.sortedBy { it.title.lowercase() }) {
             val font = if (entry.isAlias) aliasFont else entryFont
-            table.addCell(entryCell(entry.title, font))
+            table.addCell(entryCell(entry.title, font, isHighlighted = false))
-            table.addCell(entryCell(entry.pageNumber.toString(), entryFont, Element.ALIGN_RIGHT))
+            table.addCell(entryCell(entry.pageNumber.toString(), entryFont, Element.ALIGN_RIGHT, isHighlighted = highlightColumnIndex == 1))
-            for (book in refBooks) {
+            for ((i, book) in refBooks.withIndex()) {
                 val ref = entry.references[book.abbreviation]
-                table.addCell(entryCell(ref?.toString() ?: "", entryFont, Element.ALIGN_RIGHT))
+                val isHighlighted = highlightColumnIndex == 2 + i
+                table.addCell(entryCell(ref?.toString() ?: "", entryFont, Element.ALIGN_RIGHT, isHighlighted = isHighlighted))
             }
         }
 
         document.add(table)
     }
 
-    private fun headerCell(text: String, font: Font): PdfPCell {
+    private fun headerCell(text: String, font: Font, isHighlighted: Boolean): PdfPCell {
         val cell = PdfPCell(Phrase(text, font))
         cell.borderWidth = 0f
         cell.borderWidthBottom = 0.5f
         cell.paddingBottom = 4f
+        if (isHighlighted) {
+            cell.backgroundColor = highlightColor
+        }
         return cell
     }
 
-    private fun entryCell(text: String, font: Font, alignment: Int = Element.ALIGN_LEFT): PdfPCell {
+    private fun entryCell(
+        text: String,
+        font: Font,
+        alignment: Int = Element.ALIGN_LEFT,
+        isHighlighted: Boolean = false
+    ): PdfPCell {
         val cell = PdfPCell(Phrase(text, font))
         cell.borderWidth = 0f
         cell.horizontalAlignment = alignment
         cell.paddingTop = 1f
         cell.paddingBottom = 1f
+        if (isHighlighted) {
+            cell.backgroundColor = highlightColor
+        }
         return cell
     }
 }