1 files changed, 67 insertions, 0 deletions
diff --git a/viewport/viewport.go b/viewport/viewport.go
new file mode 100644
index 0000000..ff1ca05
--- /dev/null
+++ b/viewport/viewport.go
@@ -0,0 +1,67 @@
+package viewport
+
+import (
+	"strings"
+)
+
+const Caret = "⏵"
+
+func NeedsViewport(input string, height int) bool {
+	lines := strings.Split(input, "\n")
+	return len(lines)-1 > height
+}
+
+func Process(input string, height int) string {
+	lines := strings.Split(input, "\n")
+	if len(lines) > height {
+		cropped, top, bottom := Crop(input, lines, height)
+		return WithScroll(cropped, int(top*float32(height)), int(bottom*float32(height)))
+	}
+	return input
+}
+
+func Focus(lines []string, needle string) int {
+	for i, l := range lines {
+		if strings.Contains(l, needle) {
+			return i
+		}
+	}
+	return -1
+}
+
+func Crop(input string, lines []string, height int) (output string, scrollStart float32, scrollEnd float32) {
+	output = input
+	selectionIndex := Focus(lines, Caret)
+	size := len(lines)
+	if size <= height {
+		return output, 0, 1
+	}
+
+	linesAbove := height / 2
+	linesBelow := height - linesAbove
+	if linesAbove*2 < selectionIndex {
+		linesBelow++
+	}
+
+	start := selectionIndex - linesAbove
+	end := selectionIndex + linesBelow
+
+	if start < 0 {
+		end += -start
+		start = 0
+	}
+
+	if end >= size {
+		diff := size - 1 - end
+		start += diff
+		end += diff
+	}
+
+	scrollStart = float32(start) / float32(size)
+	scrollEnd = float32(end)/float32(size) + float32(1)/float32(size)
+	if scrollEnd > 1 {
+		scrollEnd = 1
+	}
+
+	return strings.Join(lines[start:end], "\n"), scrollStart, scrollEnd
+}