feat: add RTL support in epub and txt readers (#1700)

Co-authored-by: Zach Nelson <zach@zdnelson.com>
2026-05-29 03:25:17 -04:00
parent cc2079a578
commit f5bc554ae7
27 changed files with 1621 additions and 119 deletions
@@ -0,0 +1,115 @@
+#ifndef MINIBIDI_H
+#define MINIBIDI_H
+
+/*
+ * minibidi.h — standalone header for ESP32C3 BiDi calculations
+ *
+ * Derived from [mintty](https://github.com/mintty/mintty/) (Thomas Wolff, MIT licence).
+ * Stripped of: Arabic shaping, box-drawing mirror, terminal dependencies,
+ * GCC nested functions, VLAs, and non-Hebrew/English Unicode data.
+ */
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* ── Basic types ─────────────────────────────────────────────────────── */
+typedef uint8_t uchar;
+typedef uint32_t ucschar; /* Unicode codepoint; BMP-only content fits uint16_t
+                             but uint32_t is safer and ESP32C3 is 32-bit anyway */
+
+/* ── Convenience macros ──────────────────────────────────────────────── */
+#define lengthof(a) ((int)(sizeof(a) / sizeof(*(a))))
+
+/* PuTTY/mintty switch-case style — kept for readability of algorithm */
+#define when \
+  break;     \
+  case
+#define otherwise \
+  break;          \
+  default
+
+/* Maximum line length the algorithm will process.
+   Adjust to your actual screen width.  Stack cost = ~5×MAX bytes. */
+#define BIDI_MAX_LINE 128
+
+/* ── bidi_char ───────────────────────────────────────────────────────── */
+/* origwc: the codepoint as it came from the epub text stream
+   wc:     working codepoint (may be replaced by mirrored form after do_bidi)
+   index:  original logical position, so the caller can reorder glyphs */
+typedef struct {
+  ucschar origwc;
+  ucschar wc;
+  uint16_t index;
+} bidi_char;
+
+/* ── Bidi character classes (UAX #9) ────────────────────────────────── */
+enum {
+  L,   /* Left-to-Right */
+  LRE, /* Left-to-Right Embedding */
+  LRO, /* Left-to-Right Override */
+  R,   /* Right-to-Left */
+  AL,  /* Right-to-Left Arabic */
+  RLE, /* Right-to-Left Embedding */
+  RLO, /* Right-to-Left Override */
+  PDF, /* Pop Directional Format */
+  EN,  /* European Number */
+  ES,  /* European Number Separator */
+  ET,  /* European Number Terminator */
+  AN,  /* Arabic Number */
+  CS,  /* Common Number Separator */
+  NSM, /* Non-Spacing Mark */
+  BN,  /* Boundary Neutral */
+  B,   /* Paragraph Separator */
+  S,   /* Segment Separator */
+  WS,  /* Whitespace */
+  ON,  /* Other Neutrals */
+  /* Unicode 6.3 isolate types */
+  LRI, /* Left-to-Right Isolate */
+  RLI, /* Right-to-Left Isolate */
+  FSI, /* First Strong Isolate */
+  PDI, /* Pop Directional Isolate */
+};
+
+/* ── Public API ──────────────────────────────────────────────────────── */
+
+/*
+ * bidi_class(ch)
+ *   Returns the UAX#9 bidi class of Unicode codepoint ch.
+ *   Unknown characters return ON (correct per spec).
+ */
+uchar bidi_class(ucschar ch);
+
+/*
+ * is_rtl_class(bc)
+ *   Returns true if bidi class bc can cause RTL reordering.
+ *   Use to fast-skip lines with no RTL content.
+ */
+bool is_rtl_class(uchar bc);
+
+/*
+ * mirror(ch)
+ *   Returns the mirrored form of Unicode codepoint ch for UAX#9 rule L4.
+ *   If no mirror exists, returns ch unchanged.
+ */
+ucschar mirror(ucschar ch);
+
+/*
+ * do_bidi(autodir, paragraphLevel, line, count)
+ *
+ *   Applies UAX#9 Bidirectional Algorithm (rules P–L) to `line[0..count-1]`.
+ *   Reorders the array in-place; sets line[i].wc to the mirrored form where
+ *   required (rule L4).  Returns the resolved paragraph level (0=LTR, 1=RTL),
+ *   or 0 if the line was left-to-right and no reordering was done.
+ *
+ *   autodir:        true  → detect paragraph direction from content (P2/P3)
+ *                   false → use paragraphLevel as-is
+ *   paragraphLevel: 0 = LTR, 1 = RTL.  Ignored when autodir=true unless
+ *                   the content has no strong type (used as fallback).
+ *
+ *   count must be ≤ BIDI_MAX_LINE; lines longer than that are silently
+ *   truncated to BIDI_MAX_LINE before processing.
+ */
+int do_bidi(bool autodir, int paragraphLevel, bidi_char* line, int count);
+
+#endif /* MINIBIDI_H */