frontend: add a line editor for shell input

frontend/line-ed/buffer.c

@@ -0,0 +1,40 @@
+#include "buffer.h"
+#include "line-ed.h"
+
+const char *line_start(struct line_ed *ed, unsigned int y)
+{
+	const char *line = ed->l_buf;
+
+	for (unsigned int i = 0; i < y; i++) {
+		line += strcspn(line, "\n");
+
+		if (*line == '\n') {
+			line++;
+		}
+	}
+
+	return line;
+}
+
+unsigned int line_length(struct line_ed *ed, unsigned int y)
+{
+	const char *line = ed->l_buf;
+
+	for (unsigned int i = 0; i < y; i++) {
+		line += strcspn(line, "\n");
+		if (*line == '\n') {
+			line++;
+		}
+	}
+
+	if (*line == '\0') {
+		return 0;
+	}
+
+	unsigned int len = strcspn(line, "\n");
+	if (line[len] == '\n') {
+		len++;
+	}
+
+	return len;
+}