7762 - baremetal/shell: backspace

The text buffer can now shrink, which means we need to be careful to erase
the old location of the cursor. Just clear screen before render each time.
Which means we need to be more efficient with our rendering.

2 files changed, 21 insertions, 5 deletions

diff --git a/baremetal/shell/main.mu b/baremetal/shell/main.mu
index ee2dda60..262b9829 100644
--- a/baremetal/shell/main.mu
+++ b/baremetal/shell/main.mu
@@ -7,11 +7,13 @@ fn main {
   initialize-sandbox sandbox
   {
     render-sandbox 0/screen, sandbox, 2/x, 2/y
-    var key/eax: byte <- read-key 0/keyboard
-    compare key, 0
-    loop-if-=
-    # no way to quit right now; just reboot
-    edit-sandbox sandbox, key
+    {
+      var key/eax: byte <- read-key 0/keyboard
+      compare key, 0
+      loop-if-=
+      # no way to quit right now; just reboot
+      edit-sandbox sandbox, key
+    }
     loop
   }
 }
diff --git a/baremetal/shell/sandbox.mu b/baremetal/shell/sandbox.mu
index e360b9e6..4ca1082c 100644
--- a/baremetal/shell/sandbox.mu
+++ b/baremetal/shell/sandbox.mu
@@ -34,7 +34,15 @@ fn add-grapheme-to-sandbox _self: (addr sandbox), c: grapheme {
   add-grapheme-at-gap data, c
 }
 
+fn delete-grapheme-before-cursor _self: (addr sandbox) {
+  var self/esi: (addr sandbox) <- copy _self
+  var data-ah/eax: (addr handle gap-buffer) <- get self, data
+  var data/eax: (addr gap-buffer) <- lookup *data-ah
+  delete-before-gap data
+}
+
 fn render-sandbox screen: (addr screen), _self: (addr sandbox), x: int, y: int {
+  clear-screen screen
   var self/esi: (addr sandbox) <- copy _self
   var data-ah/eax: (addr handle gap-buffer) <- get self, data
   var data/eax: (addr gap-buffer) <- lookup *data-ah
@@ -43,5 +51,11 @@ fn render-sandbox screen: (addr screen), _self: (addr sandbox), x: int, y: int {
 
 fn edit-sandbox self: (addr sandbox), key: byte {
   var g/edx: grapheme <- copy key
+  {
+    compare g, 8/backspace
+    break-if-!=
+    delete-grapheme-before-cursor self
+    return
+  }
   add-grapheme-to-sandbox self, g
 }