1 files changed, 293 insertions, 0 deletions
diff --git a/drivers/tty/vt/ucs.c b/drivers/tty/vt/ucs.c
new file mode 100644
index 000000000..fc41c0bb5
--- /dev/null
+++ b/drivers/tty/vt/ucs.c
@@ -0,0 +1,293 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * ucs.c - Universal Character Set processing
+ */
+
+#include <linux/array_size.h>
+#include <linux/build_bug.h>
+#include <linux/bsearch.h>
+#include <linux/consolemap.h>
+#include <linux/math.h>
+
+struct ucs_width16 {
+	u16 first;
+	u16 last;
+};
+
+struct ucs_width32 {
+	u32 first;
+	u32 last;
+};
+
+/*
+ * Width table encoding (consumed by ucs_width_table.h):
+ *
+ * Zero- and double-width ranges are merged into one sorted-by-`first` table
+ * per region (BMP / non-BMP). The BMP table stores plain (first, last)
+ * pairs; per-entry width lives in a packed bitmap *hosted by the non-BMP
+ * table*.
+ *
+ * That hosting is the whole point of the encoding. Non-BMP code points use
+ * only 20 bits, so each u32 has 12 spare high bits sitting around doing
+ * nothing — we'd rather use them than spend a separate parallel array for
+ * width and BMP-bitmap bits. So we move the cp value up by UCS_CP_SHIFT
+ * and stash metadata in the now-free low bits of `last`:
+ *   - bit UCS_NONBMP_W2_FLAG_BIT: this entry's own width (0=zero, 1=double),
+ *   - bits 0..UCS_NONBMP_BMP_BITS-1: a chunk of the BMP double-width
+ *     bitmap. Bit `j` of the chunk in non-BMP entry `c` is set iff BMP
+ *     entry (c * UCS_NONBMP_BMP_BITS + j) is double-width. The first
+ *     ceil(N_BMP / UCS_NONBMP_BMP_BITS) non-BMP entries carry the bitmap;
+ *     the rest leave these bits zero.
+ *
+ * Because the metadata bits sit strictly below the lowest cp-scale bit,
+ * the bsearch comparator does plain u32 comparison on the shifted key and
+ * stored values without masking — ordering between distinct code points is
+ * undisturbed.
+ */
+#define UCS_CP_SHIFT           12
+#define UCS_NONBMP_W2_FLAG_BIT 11
+#define UCS_NONBMP_W2_FLAG     (1u << UCS_NONBMP_W2_FLAG_BIT)
+
+#define BMP_0WIDTH(first, last)   first, last
+#define BMP_2WIDTH(first, last)   first, last
+#define RANGE_0WIDTH(first, last) \
+	(u32)(first) << UCS_CP_SHIFT,  (u32)(last) << UCS_CP_SHIFT
+#define RANGE_2WIDTH(first, last) \
+	(u32)(first) << UCS_CP_SHIFT, ((u32)(last) << UCS_CP_SHIFT) | UCS_NONBMP_W2_FLAG
+#define BMP_2W_BITS(b)            (b)
+
+#include "ucs_width_table.h"
+
+static_assert(UCS_NONBMP_BMP_BITS <= UCS_NONBMP_W2_FLAG_BIT,
+	      "BMP bitmap chunk would overlap the per-entry width flag");
+static_assert(UCS_NONBMP_W2_FLAG_BIT < UCS_CP_SHIFT,
+	      "Metadata bits collide with the shifted cp value");
+static_assert(DIV_ROUND_UP(ARRAY_SIZE(ucs_bmp_ranges), UCS_NONBMP_BMP_BITS)
+	      <= ARRAY_SIZE(ucs_nonbmp_ranges),
+	      "Not enough non-BMP entries to host the BMP width bitmap");
+
+#define UCS_IS_BMP(cp)	((cp) <= 0xffff)
+
+static int width16_cmp(const void *key, const void *element)
+{
+	u16 cp = *(u16 *)key;
+	const struct ucs_width16 *entry = element;
+
+	if (cp < entry->first)
+		return -1;
+	if (cp > entry->last)
+		return 1;
+	return 0;
+}
+
+static int width32_cmp(const void *key, const void *element)
+{
+	u32 k = *(u32 *)key;
+	const struct ucs_width32 *entry = element;
+
+	if (k < entry->first)
+		return -1;
+	if (k > entry->last)
+		return 1;
+	return 0;
+}
+
+/**
+ * ucs_get_width() - Get the display width of a Unicode code point.
+ * @cp: Unicode code point (UCS-4)
+ *
+ * Return: 2 for double-width (East Asian Wide/Fullwidth, emoji, ...),
+ *         0 for zero-width (combining marks, format characters, ...),
+ *         1 for everything else (the common case).
+ */
+unsigned int ucs_get_width(u32 cp)
+{
+	const struct ucs_width16 *e16;
+	const struct ucs_width32 *e32;
+	unsigned int idx;
+	u32 k;
+
+	if (UCS_IS_BMP(cp)) {
+		u16 bmp = cp;
+
+		if (bmp < ucs_bmp_ranges[0].first ||
+		    bmp > ucs_bmp_ranges[ARRAY_SIZE(ucs_bmp_ranges) - 1].last)
+			return 1;
+
+		e16 = __inline_bsearch(&bmp, ucs_bmp_ranges,
+				       ARRAY_SIZE(ucs_bmp_ranges),
+				       sizeof(*ucs_bmp_ranges), width16_cmp);
+		if (!e16)
+			return 1;
+
+		idx = e16 - ucs_bmp_ranges;
+		return (ucs_nonbmp_ranges[idx / UCS_NONBMP_BMP_BITS].last
+			>> (idx % UCS_NONBMP_BMP_BITS)) & 1 ? 2 : 0;
+	}
+
+	k = cp << UCS_CP_SHIFT;
+	if (k < ucs_nonbmp_ranges[0].first ||
+	    k > ucs_nonbmp_ranges[ARRAY_SIZE(ucs_nonbmp_ranges) - 1].last)
+		return 1;
+
+	e32 = __inline_bsearch(&k, ucs_nonbmp_ranges,
+			       ARRAY_SIZE(ucs_nonbmp_ranges),
+			       sizeof(*ucs_nonbmp_ranges), width32_cmp);
+	if (!e32)
+		return 1;
+	return (e32->last & UCS_NONBMP_W2_FLAG) ? 2 : 0;
+}
+
+/*
+ * Structure for base with combining mark pairs and resulting recompositions.
+ * Using u16 to save space since all values are within BMP range.
+ */
+struct ucs_recomposition {
+	u16 base;	/* base character */
+	u16 mark;	/* combining mark */
+	u16 recomposed;	/* corresponding recomposed character */
+};
+
+#include "ucs_recompose_table.h"
+
+struct compare_key {
+	u16 base;
+	u16 mark;
+};
+
+static int recomposition_cmp(const void *key, const void *element)
+{
+	const struct compare_key *search_key = key;
+	const struct ucs_recomposition *entry = element;
+
+	/* Compare base character first */
+	if (search_key->base < entry->base)
+		return -1;
+	if (search_key->base > entry->base)
+		return 1;
+
+	/* Base characters match, now compare combining character */
+	if (search_key->mark < entry->mark)
+		return -1;
+	if (search_key->mark > entry->mark)
+		return 1;
+
+	/* Both match */
+	return 0;
+}
+
+/**
+ * ucs_recompose() - Attempt to recompose two Unicode characters into a single character.
+ * @base: Base Unicode code point (UCS-4)
+ * @mark: Combining mark Unicode code point (UCS-4)
+ *
+ * Return: Recomposed Unicode code point, or 0 if no recomposition is possible
+ */
+u32 ucs_recompose(u32 base, u32 mark)
+{
+	/* Check if characters are within the range of our table */
+	if (base < UCS_RECOMPOSE_MIN_BASE || base > UCS_RECOMPOSE_MAX_BASE ||
+	    mark < UCS_RECOMPOSE_MIN_MARK || mark > UCS_RECOMPOSE_MAX_MARK)
+		return 0;
+
+	struct compare_key key = { base, mark };
+	struct ucs_recomposition *result =
+		__inline_bsearch(&key, ucs_recomposition_table,
+				 ARRAY_SIZE(ucs_recomposition_table),
+				 sizeof(*ucs_recomposition_table),
+				 recomposition_cmp);
+
+	return result ? result->recomposed : 0;
+}
+
+/*
+ * The fallback table structures implement a 2-level lookup.
+ */
+
+struct ucs_page_desc {
+	u8 page;	/* Page index (high byte of code points) */
+	u8 count;	/* Number of entries in this page */
+	u16 start;	/* Start index in entries array */
+};
+
+struct ucs_page_entry {
+	u8 offset;	/* Offset within page (0-255) */
+	u8 fallback;	/* Fallback character or range start marker */
+};
+
+#include "ucs_fallback_table.h"
+
+static int ucs_page_desc_cmp(const void *key, const void *element)
+{
+	u8 page = *(u8 *)key;
+	const struct ucs_page_desc *entry = element;
+
+	if (page < entry->page)
+		return -1;
+	if (page > entry->page)
+		return 1;
+	return 0;
+}
+
+static int ucs_page_entry_cmp(const void *key, const void *element)
+{
+	u8 offset = *(u8 *)key;
+	const struct ucs_page_entry *entry = element;
+
+	if (offset < entry->offset)
+		return -1;
+	if (entry->fallback == UCS_PAGE_ENTRY_RANGE_MARKER) {
+		if (offset > entry[1].offset)
+			return 1;
+	} else {
+		if (offset > entry->offset)
+			return 1;
+	}
+	return 0;
+}
+
+/**
+ * ucs_get_fallback() - Get a substitution for the provided Unicode character
+ * @cp: Unicode code point (UCS-4)
+ *
+ * Get a simpler fallback character for the provided Unicode character.
+ * This is used for terminal display when corresponding glyph is unavailable.
+ * The substitution may not be as good as the actual glyph for the original
+ * character but still way more helpful than a squared question mark.
+ *
+ * Return: Fallback Unicode code point, or 0 if none is available
+ */
+u32 ucs_get_fallback(u32 cp)
+{
+	const struct ucs_page_desc *page;
+	const struct ucs_page_entry *entry;
+	u8 page_idx = cp >> 8, offset = cp;
+
+	if (!UCS_IS_BMP(cp))
+		return 0;
+
+	/*
+	 * Full-width to ASCII mapping (covering all printable ASCII 33-126)
+	 * 0xFF01 (！) to 0xFF5E (～) -> ASCII 33 (!) to 126 (~)
+	 * We process them programmatically to reduce the table size.
+	 */
+	if (cp >= 0xFF01 && cp <= 0xFF5E)
+		return cp - 0xFF01 + 33;
+
+	page = __inline_bsearch(&page_idx, ucs_fallback_pages,
+				ARRAY_SIZE(ucs_fallback_pages),
+				sizeof(*ucs_fallback_pages),
+				ucs_page_desc_cmp);
+	if (!page)
+		return 0;
+
+	entry = __inline_bsearch(&offset, ucs_fallback_entries + page->start,
+				 page->count, sizeof(*ucs_fallback_entries),
+				 ucs_page_entry_cmp);
+	if (!entry)
+		return 0;
+
+	if (entry->fallback == UCS_PAGE_ENTRY_RANGE_MARKER)
+		entry++;
+	return entry->fallback;
+}