Add option to highlight specific lines

The highlighting is done by applying a grey background with 50% opacity
to all selected lines.

The parsing of the option is as follows:
    - Each input (lines or line ranges) is separated by ;
    - To provide a range, "-" separates the start and the end of the range.
    - In a range, both start and end are required, and end must be greater than start.

config.go

@@ -3,9 +3,12 @@ package main
 import (
 	"embed"
 	"encoding/json"
+	"fmt"
 	"io/fs"
 	"os"
 	"path/filepath"
+	"strconv"
+	"strings"
 	"time"
 
 	"github.com/adrg/xdg"
@@ -47,9 +50,64 @@ type Config struct {
 	// Line
 	LineHeight      float64 `json:"line_height" help:"Line height relative to font size." group:"Line" placeholder:"1.2"`
 	Lines           []int   `json:"-" help:"Lines to capture (start,end)." group:"Line" placeholder:"0,-1" value:"0,-1"`
+	HighlightLines  string  `json:"-" help:"Lines to highlight (range: \"start-end\", separator:\";\")." group:"Line" placeholder:"0,10" value:""`
 	ShowLineNumbers bool    `json:"show_line_numbers" help:"" group:"Line" placeholder:"false"`
 }
 
+// ComputeHighlightedLines parse the config.HighlightLines option
+// And return a map of Line numbers where the highlight should be applied
+func (cfg Config) ComputeHighlightedLines() map[int]bool {
+	uniqueNumbers := make(map[int]bool) // Use a map to ensure uniqueness
+	if cfg.HighlightLines == "" {
+		return uniqueNumbers
+	}
+	// Split the input by ';'
+	parts := strings.Split(cfg.HighlightLines, ";")
+
+	for _, part := range parts {
+		// Check if the part contains a dash '-'
+		if strings.Contains(part, "-") {
+			// Split the part by '-' to get start and end of the range
+			rangeParts := strings.Split(part, "-")
+			if len(rangeParts) == 2 {
+				start, err1 := strconv.Atoi(rangeParts[0])
+				end, err2 := strconv.Atoi(rangeParts[1])
+
+				if end <= start {
+					err := fmt.Errorf("end of range lower (%d) than start of range (%d): %s", end, start, part)
+					printErrorFatal("error while parsing highlight lines range", err)
+				}
+				// If parsing is successful and start <= end
+				if err1 == nil && err2 == nil && start <= end {
+					// Add all numbers in the range to the map
+					for i := start; i <= end; i++ {
+						uniqueNumbers[i] = true
+					}
+				} else if err1 != nil {
+					err := fmt.Errorf("unable to parse the first part of the range: %s", rangeParts[0])
+					printErrorFatal("error while parsing highlight lines range", err)
+				} else if err2 != nil {
+					err := fmt.Errorf("unable to parse the second part of the range: %s", rangeParts[1])
+					printErrorFatal("error while parsing highlight lines range", err)
+				}
+			} else {
+				err := fmt.Errorf("a range should contains exactly two part: %s", part)
+				printErrorFatal("error while parsing highlight lines range", err)
+			}
+		} else {
+			// If no dash, just convert the number and add it to the map
+			num, err := strconv.Atoi(part)
+			if err != nil {
+				err := fmt.Errorf("unable to parse to integer: %s", part)
+				printErrorFatal("error while parsing highlight lines", err)
+			}
+			uniqueNumbers[num] = true
+		}
+	}
+
+	return uniqueNumbers
+}
+
 // Shadow is the configuration options for a drop shadow.
 type Shadow struct {
 	Blur float64 `json:"blur" help:"Shadow Gaussian Blur." placeholder:"0"`

main.go

@@ -4,9 +4,11 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"math"
 	"os"
 	"path/filepath"
 	"runtime/debug"
+	"strconv"
 	"strings"
 
 	"github.com/alecthomas/chroma/v2"
@@ -171,6 +173,7 @@ func main() {
 	for i := range config.Lines {
 		config.Lines[i]--
 	}
+	highlightedLines := config.ComputeHighlightedLines()
 
 	var strippedInput string = ansi.Strip(input)
 	isAnsi := strings.ToLower(config.Language) == "ansi" || strippedInput != input
@@ -344,10 +347,29 @@ func main() {
 		}
 		// Offset the text by padding...
 		// (x, y) -> (x+p, y+p)
+
+		var bg *etree.Element
+		doHighlightLine := highlightedLines[i+1+offsetLine-softWrapOffset]
+		if config.SoftWrap {
+			// If the current line is soft-wrapped, we need to find the previous real line
+			if !isRealLine[i] {
+				j := i
+				// iterate previous lines until we find the previous real line
+				for ; !isRealLine[j]; j-- {
+				}
+				// we apply to the current line the highlight status of the found real line
+				doHighlightLine = highlightedLines[j+1+offsetLine-softWrapOffset]
+			}
+		}
+
 		if config.ShowLineNumbers {
 			ln := etree.NewElement("tspan")
 			ln.CreateAttr("xml:space", "preserve")
-			ln.CreateAttr("fill", s.Get(chroma.LineNumbers).Colour.String())
+			if doHighlightLine {
+				ln.CreateAttr("fill", s.Get(chroma.LineHighlight).Colour.String())
+			} else {
+				ln.CreateAttr("fill", s.Get(chroma.LineNumbers).Colour.String())
+			}
 			if config.SoftWrap {
 				if (isAnsi && strippedIsRealLine[i]) || (!isAnsi && isRealLine[i]) {
 					ln.SetText(fmt.Sprintf("%3d  ", i+1+offsetLine-softWrapOffset))
@@ -357,14 +379,33 @@ func main() {
 			} else {
 				ln.SetText(fmt.Sprintf("%3d  ", i+1+offsetLine))
 			}
+			ln.CreateAttr("height", strconv.Itoa(int(math.Round(config.Font.Size*config.LineHeight))))
 			line.InsertChildAt(0, ln)
 		}
 		if config.SoftWrap && !((isAnsi && strippedIsRealLine[i]) || (!isAnsi && isRealLine[i])) {
 			softWrapOffset++
 		}
 		x := float64(config.Padding[left] + config.Margin[left])
-		y := (float64(i+1))*(config.Font.Size*config.LineHeight) + float64(config.Padding[top]) + float64(config.Margin[top])
-
+		// Rounding required to ensure that each line have the same height
+		y := (float64(i+1))*math.Round(config.Font.Size*config.LineHeight) + float64(config.Padding[top]) + float64(config.Margin[top])
+		if doHighlightLine {
+			// Create a background element, with grey color and 50% opacity
+			bg = etree.NewElement("rect")
+			bg.CreateAttr("fill", "grey")
+			bg.CreateAttr("fill-opacity", "0.5")
+			// This lineWidth is not accurate when the width is dynamic, it will be computed later
+			lineWidth := imageWidth + config.Margin[left] + config.Padding[left] + config.Margin[right] + config.Padding[right]
+			bg.CreateAttr("width", strconv.Itoa(int(lineWidth)))
+			// We round to ensure that two highlighted consecutive lines do not leave a one pixel line between
+			bg.CreateAttr("height", strconv.Itoa(int(math.Round(config.Font.Size*config.LineHeight))))
+			line.Parent().InsertChildAt(0, bg)
+
+			yRect := float64(i)*math.Round(config.Font.Size*config.LineHeight) +
+				config.Padding[top] +
+				config.Margin[top] + math.Round(config.LineHeight*config.Font.Size)/4
+			// We round to ensure that two highlighted consecutive lines do not leave a one pixel line between
+			svg.Move(bg, 0, math.Round(yRect))
+		}
 		svg.Move(line, x, y)
 
 		// We are passed visible lines, remove the rest.
@@ -402,6 +443,13 @@ func main() {
 		}
 	}
 
+	// Adjust the highlighted rect width with the accurate computed width
+	if len(highlightedLines) != 0 {
+		for _, elem := range textGroup.SelectElements("rect") {
+			elem.CreateAttr("width", strconv.Itoa(int(imageWidth)))
+		}
+	}
+
 	if !autoHeight || !autoWidth {
 		svg.AddClipPath(image, "terminalMask",
 			config.Margin[left], config.Margin[top],