From 73e59188a7fb70d83ae8614d90fe2193dff5bcf8 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Mon, 28 May 2018 23:25:21 -0700
Subject: [PATCH] indenter

---
 brag/brag/codegen/reader.rkt   |  1 +
 brag/brag/private/indenter.rkt | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 brag/brag/private/indenter.rkt

diff --git a/brag/brag/codegen/reader.rkt b/brag/brag/codegen/reader.rkt
index c4f35f4..36dd5db 100755
--- a/brag/brag/codegen/reader.rkt
+++ b/brag/brag/codegen/reader.rkt
@@ -61,6 +61,7 @@ brag/codegen/sexp-based-lang
 (define (my-get-info key default default-filter)
   (case key
     [(color-lexer) (dynamic-require 'brag/private/colorer 'color-brag (λ () #f))]
+    [(drracket:indentation) (dynamic-require 'brag/private/indenter 'indent-brag (λ () #f))]
     [else
      (default-filter key default)]))
 
diff --git a/brag/brag/private/indenter.rkt b/brag/brag/private/indenter.rkt
new file mode 100644
index 0000000..bec7b8c
--- /dev/null
+++ b/brag/brag/private/indenter.rkt
@@ -0,0 +1,32 @@
+#lang racket/base
+(require racket/class)
+(provide indent-brag)
+
+(define (line text pos)
+  (send text position-line pos))
+
+(define (previous-line text pos)
+  (define this-line (line text pos))
+  (and (positive? this-line) (sub1 this-line)))
+
+(define (line-first-visible-char text line)
+  (define (char text pos) (and pos (send text get-character pos)))
+  (char text (for*/first ([pos (in-range (send text line-start-position line)
+                                         (send text line-end-position line))]
+                          [c (in-value (char text pos))]
+                          #:unless (char-blank? c))
+               pos)))
+
+(define (indent-brag tbox [posn 0])
+  (define prev-line (previous-line tbox posn))
+  (define this-line (line tbox posn))
+  (cond
+    [(not prev-line) #f]
+    [(char=? (line-first-visible-char tbox this-line) #\|)
+     (define start (send tbox line-start-position prev-line))
+     (define end (send tbox line-end-position prev-line))
+     (for*/first ([pos (in-range start end)]
+                  [c (in-value (send tbox get-character pos))]
+                  #:when (or (char=? c #\:) (char=? c #\|)))
+       (- pos start))]
+    [else #f]))
\ No newline at end of file