fix(pager): memory/cpu usage when using soft-wrap (#827)

easily reproducible, especially with something like

```sh
PAGER="gum pager" man ls
```

Problems here were:
- reflow's truncate is not very efficient
- useless calls to truncate
- bad strings.Replace calls (not necessary, and wouldn't work if text has ansi sequences)

go.mod

@@ -14,7 +14,6 @@ require (
 	github.com/charmbracelet/x/ansi v0.8.0
 	github.com/charmbracelet/x/editor v0.1.0
 	github.com/charmbracelet/x/term v0.2.1
-	github.com/muesli/reflow v0.3.0
 	github.com/muesli/roff v0.1.0
 	github.com/muesli/termenv v0.15.3-0.20241211131612-0d230cb6eb15
 	github.com/rivo/uniseg v0.4.7
@@ -40,6 +39,7 @@ require (
 	github.com/muesli/ansi v0.0.0-20230316100256-276c6243b2f6 // indirect
 	github.com/muesli/cancelreader v0.2.2 // indirect
 	github.com/muesli/mango v0.2.0 // indirect
+	github.com/muesli/reflow v0.3.0 // indirect
 	github.com/yuin/goldmark v1.7.4 // indirect
 	github.com/yuin/goldmark-emoji v1.0.4 // indirect
 	golang.org/x/exp v0.0.0-20240613232115-7f521ea00fb8 // indirect

pager/pager.go

@@ -13,7 +13,7 @@ import (
 	"github.com/charmbracelet/bubbles/viewport"
 	tea "github.com/charmbracelet/bubbletea"
 	"github.com/charmbracelet/lipgloss"
-	"github.com/muesli/reflow/truncate"
+	"github.com/charmbracelet/x/ansi"
 )
 
 type keymap struct {
@@ -146,17 +146,21 @@ func (m *model) processText(msg tea.WindowSizeMsg) {
 		if m.showLineNumbers {
 			text.WriteString(m.lineNumberStyle.Render(fmt.Sprintf("%4d │ ", i+1)))
 		}
-		for m.softWrap && lipgloss.Width(line) > m.maxWidth {
-			truncatedLine := truncate.String(line, uint(m.maxWidth)) //nolint: gosec
-			text.WriteString(textStyle.Render(truncatedLine))
-			text.WriteString("\n")
-			if m.showLineNumbers {
-				text.WriteString(m.lineNumberStyle.Render("     │ "))
+		idx := 0
+		if w := ansi.StringWidth(line); m.softWrap && w > m.maxWidth {
+			for w > idx {
+				if m.showLineNumbers && idx != 0 {
+					text.WriteString(m.lineNumberStyle.Render("     │ "))
+				}
+				truncatedLine := ansi.Cut(line, idx, m.maxWidth+idx)
+				idx += m.maxWidth
+				text.WriteString(textStyle.Render(truncatedLine))
+				text.WriteString("\n")
 			}
-			line = strings.Replace(line, truncatedLine, "", 1)
+		} else {
+			text.WriteString(textStyle.Render(line)) //nolint: gosec
+			text.WriteString("\n")
 		}
-		text.WriteString(textStyle.Render(truncate.String(line, uint(m.maxWidth)))) //nolint: gosec
-		text.WriteString("\n")
 	}
 
 	diffHeight := m.viewport.Height - lipgloss.Height(text.String())

pager/search.go

@@ -8,7 +8,7 @@ import (
 	"github.com/charmbracelet/bubbles/textinput"
 	"github.com/charmbracelet/gum/internal/utils"
 	"github.com/charmbracelet/lipgloss"
-	"github.com/muesli/reflow/truncate"
+	"github.com/charmbracelet/x/ansi"
 )
 
 type search struct {
@@ -150,14 +150,18 @@ func (s *search) PrevMatch(m *model) {
 func softWrapEm(str string, maxWidth int, softWrap bool) string {
 	var text strings.Builder
 	for _, line := range strings.Split(str, "\n") {
-		for softWrap && lipgloss.Width(line) > maxWidth {
-			truncatedLine := truncate.String(line, uint(maxWidth)) //nolint: gosec
-			text.WriteString(truncatedLine)
+		idx := 0
+		if w := ansi.StringWidth(line); softWrap && w > maxWidth {
+			for w > idx {
+				truncatedLine := ansi.Cut(line, idx, maxWidth+idx)
+				idx += maxWidth
+				text.WriteString(truncatedLine)
+				text.WriteString("\n")
+			}
+		} else {
+			text.WriteString(line) //nolint: gosec
 			text.WriteString("\n")
-			line = strings.Replace(line, truncatedLine, "", 1)
 		}
-		text.WriteString(truncate.String(line, uint(maxWidth))) //nolint: gosec
-		text.WriteString("\n")
 	}
 
 	return text.String()