A line address past the last newline is sam's empty line there

Forward line addresses followed acme's number(), which refuses the empty
line after a text's last newline, so 1 of an empty buffer and 2 of "a\n"
were out of range and Edit 1i/header/ failed on an empty file. They now
follow sam's lineaddr (editors/sam/address.c:180-215): #0,#0 and #2,#2.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

2 files changed, 67 insertions, 20 deletions

diff --git a/docs/fs.md b/docs/fs.md
index d1be9f6a..8397ed88 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -483,6 +483,12 @@ pane) and `exec` alike; an unclosed group is refused (``unmatched `{'``).
 bash's builtin `printf` writes a line at a time, so write a block with a
 heredoc or `env printf`.
 
+A line number counts newlines as sam's lineaddr does (editors/sam/
+address.c:180), so the empty line just past a text's last newline is an
+address: `1` of an empty text is `#0,#0`, `2` of `a\n` is `#2,#2`, and
+`Edit 1i/header/` on an empty file inserts; a line past that is `address
+out of range`.
+
 `line:col` is a pardes extension to sam's addresses, the spelling Look
 takes in `file:12:5`: `12:5` is the point at line 12, column 5, and it
 composes like any simple address (`12:5,14:1`, `12:5+#3`). The column is
diff --git a/src/ninep/addr.zig b/src/ninep/addr.zig
index 1e47183b..e1d6da0f 100644
--- a/src/ninep/addr.zig
+++ b/src/ninep/addr.zig
@@ -188,30 +188,43 @@ pub const Addr = struct {
                 while (q0 > 0 and a.text[q0 - 1] != '\n') q0 -= 1;
                 return .{ .q0 = clip(q0), .q1 = clip(q1) };
             },
-            '+' => {
-                if (q1 > 0) while (q1 < a.text.len and a.text[q1 - 1] != '\n') {
-                    q1 += 1;
-                };
-                q0 = q1;
-            },
-            else => {
-                q0 = 0;
-                q1 = 0;
-            },
+            else => {},
         }
-        while (line > 0 and q1 < a.text.len) {
-            const ch = a.text[q1];
-            q1 += 1;
-            if (ch == '\n' or q1 == a.text.len) {
-                line -= 1;
-                if (line > 0) q0 = q1;
+        // Forward, sam's lineaddr (editors/sam/address.c:180-215), not acme's
+        // number(): line n is found by counting newlines, and a line just
+        // past the last newline is an empty one there, so `1` of an empty
+        // text is #0,#0 and `2` of "a\n" is #2,#2; only past that is it out
+        // of range.
+        const nc = a.text.len;
+        var p: usize = 0;
+        var p1: usize = 0;
+        if (n == 0) {
+            if (dir == 0 or q1 == 0) return .{ .q0 = 0, .q1 = 0 };
+            p1 = q1;
+            p = q1 - 1;
+        } else {
+            var k: u32 = 1;
+            if (dir != 0 and q1 > 0) {
+                p = q1 - 1;
+                k = @intFromBool(a.text[p] == '\n');
+                p += 1;
+            }
+            while (k < n) {
+                if (p >= nc) {
+                    a.err = e_range;
+                    return null;
+                }
+                if (a.text[p] == '\n') k += 1;
+                p += 1;
             }
+            p1 = p;
         }
-        if (line > 0) {
-            a.err = e_range;
-            return null;
+        while (p < nc) {
+            const ch = a.text[p];
+            p += 1;
+            if (ch == '\n') break;
         }
-        return .{ .q0 = clip(q0), .q1 = clip(q1) };
+        return .{ .q0 = clip(p1), .q1 = clip(p) };
     }
 
     /// acme's regexp(): forward from the end of `r` to the limit, wrapping
@@ -476,3 +489,31 @@ test "the address language, form by form" {
     _ = th.wr(p, addr, "#0");
     try testing.expectEqual(E.INVAL, th.wr(p, addr, nested).errno());
 }
+
+test "a line just past the last newline is an empty one there, as sam's lineaddr has it" {
+    const Case = struct { text: []const u8, expr: []const u8, q0: u32, q1: u32 };
+    for ([_]Case{
+        .{ .text = "", .expr = "1", .q0 = 0, .q1 = 0 },
+        .{ .text = "a\n", .expr = "2", .q0 = 2, .q1 = 2 },
+        .{ .text = "a\nb", .expr = "2", .q0 = 2, .q1 = 3 },
+        .{ .text = "a\nb\n", .expr = "1", .q0 = 0, .q1 = 2 },
+        .{ .text = "a\nb\n", .expr = "0", .q0 = 0, .q1 = 0 },
+        .{ .text = "a\nb\n", .expr = "1+1", .q0 = 2, .q1 = 4 },
+        .{ .text = "a\nb\n", .expr = "$", .q0 = 4, .q1 = 4 },
+    }) |c| {
+        var a: Addr = .{ .text = c.text, .lim = null, .expr = c.expr };
+        const r = a.address(.{ .q0 = 0, .q1 = 0 }).?;
+        try testing.expectEqual(c.q0, r.q0);
+        try testing.expectEqual(c.q1, r.q1);
+    }
+    for ([_][2][]const u8{ .{ "", "2" }, .{ "a\n", "3" } }) |c| {
+        var a: Addr = .{ .text = c[0], .lim = null, .expr = c[1] };
+        try testing.expectEqual(null, a.address(.{ .q0 = 0, .q1 = 0 }));
+        try testing.expectEqualStrings(e_range, a.err);
+    }
+    // Edit's addresses are these: 1i on an empty file inserts.
+    const p = try th.withFile(testing.allocator, "");
+    defer p.deinit();
+    try testing.expectEqual(Status.ok, th.wr(p, Node.of(th.serialOf(p), .ctl), "Edit 1i/header/\n").reply.status);
+    try testing.expectEqualStrings("header", p.panes[0].?.file.?.content);
+}