qrcode: Materialise each mode segment once when the mode changes

The mode sequence derivation extended the pending segment's data array
per character, rebuilding it via aload each time the character class
run breaks without a mode change - quadratic whenever a long segment
accumulates single-character steps, as byte mode does over mixed-case
text and URLs. Every segment is a contiguous slice of the message, so
track only its start position and take a single getinterval when the
mode changes or the message ends.

Author: terryburton

CHANGES

@@ -1,3 +1,8 @@
+XXXX-XX-XX
+
+* The performance of the QR Code encoder was improved for data that mixes character classes, such as mixed-case text and URLs, by materialising each mode segment once when the mode changes rather than extending it character by character.
+
+
 2026-07-06
 
 * The performance of the DotCode encoder was improved by sharing a single codeword placement walk between the mask candidates, by deriving each candidate's error correction codewords from those of the unmasked message, and by scoring each candidate together with its lit-edge variant in a single evaluation pass.

src/qrcode.ps.src

@@ -1080,7 +1080,7 @@ begin
         %
         % Derive optimal sequence using stack-based accumulation
         %
-        /mode -1 def /seqok true def mark /i 0 def {
+        /mode -1 def /seqok true def /i 0 def /segstart -1 def mark {
             i msglen ge {exit} if
             /numK numKs i get def
             /numB numBs i get def
@@ -1160,23 +1160,25 @@ begin
                 } if
             } loop
             dup //qrcode.K eq fnc1first and {pop //qrcode.B} if  % No kanji with fnc1first
-            dup mode eq {  % Same mode encode directly - extend data on stack
+            dup mode eq {  % Same mode - grow the pending segment, deferring extraction
                 pop
-                /dat msg i mode //qrcode.K eq {2} {1} ifelse getinterval def
-                [ exch aload pop dat aload pop ]  % Extend top data array with dat
-            } {  % Change mode - push mode and data to stack
-                /mode exch def
-                mode //qrcode.K eq {//qrcode.K msg i numK 2 mul getinterval} if
-                mode //qrcode.B eq {//qrcode.B msg i numB getinterval} if
-                mode //qrcode.A eq {//qrcode.A msg i numA getinterval} if
-                mode //qrcode.N eq {//qrcode.N msg i numN getinterval} if
-                mode //qrcode.E eq {//qrcode.E msg i 1 getinterval  /mode -1 def} if
-                /dat 1 index def  % Copy data to dat variable (sw and data stay on stack)
+                /i i mode //qrcode.K eq {2} {1} ifelse add def
+            } {  % Change mode - materialise the pending segment beneath the new mode, then start a new one
+                segstart 0 ge {msg segstart i segstart sub getinterval exch} if
+                dup /mode exch def
+                /segstart i def
+                mode //qrcode.K eq {/i i numK 2 mul add def} if
+                mode //qrcode.B eq {/i i numB add def} if
+                mode //qrcode.A eq {/i i numA add def} if
+                mode //qrcode.N eq {/i i numN add def} if
+                mode //qrcode.E eq {/i i 1 add def  /mode -1 def} if
             } ifelse
-            /i i dat length add def
         } loop
-        % Build seq from stack (or set to -1 if invalid)
-        seqok {counttomark array astore exch pop} {cleartomark -1} ifelse /seq exch def
+        % Build seq from stack, materialising the final pending segment (or set to -1 if invalid)
+        seqok {
+            segstart 0 ge {msg segstart i segstart sub getinterval} if
+            counttomark array astore exch pop
+        } {cleartomark -1} ifelse /seq exch def
 
         %
         % Encode the sequence