feat: closest-pt size selection instead of ordinal slot (#1912)

Add SdCardFontFamilyInfo::pickClosestSize(targetPtSize) and have the
manager and SdCardFontSystem drive size selection from a target point
size derived from the user's font-size enum (SMALL=12, MEDIUM=14,
LARGE=16, EXTRA_LARGE=18) rather than indexing the family's sorted size
list by enum ordinal.

The ordinal-slot approach mis-selects whenever a family doesn't ship the
canonical {12,14,16,18} set: a family with only [10,14,18] would map
SMALL/MEDIUM/LARGE/EXTRA_LARGE to 10/14/18/18 — fine for SMALL but
arbitrary for the rest. Closest-pt always picks the on-disk file nearest
to the user-intended point size, with a deterministic smaller-pt
tie-break.

No change for canonical-sized families.

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

---

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>**_

lib/EpdFont/SdCardFontManager.cpp

@@ -28,25 +28,21 @@ int SdCardFontManager::computeFontId(uint32_t contentHash, const char* familyNam
   return id != 0 ? id : 1;  // 0 is reserved as "not found" sentinel
 }
 
-bool SdCardFontManager::loadFamily(const SdCardFontFamilyInfo& family, GfxRenderer& renderer, uint8_t fontSizeEnum) {
+bool SdCardFontManager::loadFamily(const SdCardFontFamilyInfo& family, GfxRenderer& renderer, uint8_t targetPtSize) {
   // Unload any previously loaded family first
   if (!loadedFamilyName_.empty()) {
     unloadAll(renderer);
   }
 
-  // Select by ordinal position: sort available sizes, then map the font size
+  // Pick the single file whose size is closest to targetPtSize. Loading
-  // enum (SMALL=0 .. EXTRA_LARGE=3) to the corresponding slot. When the
+  // only one size bounds resident memory (intervals + kern/ligature tables
-  // family has fewer sizes than 4, clamp to the last available size.
+  // per style) to one file's worth, vs. N_sizes × per-file overhead.
-  auto sizes = family.availableSizes();
+  const SdCardFontFileInfo* selected = family.pickClosestSize(targetPtSize);
-  if (sizes.empty()) {
+  if (!selected) {
     LOG_ERR("SDMGR", "Family %s has no files to load", family.name.c_str());
     return false;
   }
 
-  uint8_t idx = fontSizeEnum;
-  if (idx >= sizes.size()) idx = sizes.size() - 1;
-  const SdCardFontFileInfo* selected = family.findFile(sizes[idx]);
-
   auto* font = new (std::nothrow) SdCardFont();
   if (!font) {
     LOG_ERR("SDMGR", "Failed to allocate SdCardFont for %s", selected->path.c_str());
@@ -70,8 +66,8 @@ bool SdCardFontManager::loadFamily(const SdCardFontFamilyInfo& family, GfxRender
   renderer.registerSdCardFont(fontId, font);
   loaded_.push_back({font, fontId, selected->pointSize});
 
-  LOG_DBG("SDMGR", "Loaded %s size=%u id=%d styles=%u (sizeEnum=%u)", selected->path.c_str(), selected->pointSize,
+  LOG_DBG("SDMGR", "Loaded %s size=%u id=%d styles=%u (target=%u)", selected->path.c_str(), selected->pointSize, fontId,
-          fontId, font->styleCount(), fontSizeEnum);
+          font->styleCount(), targetPtSize);
 
   EpdFontFamily fontFamily(font->getEpdFont(0), font->getEpdFont(1), font->getEpdFont(2), font->getEpdFont(3));
   renderer.insertFont(fontId, fontFamily);

lib/EpdFont/SdCardFontManager.h

@@ -15,12 +15,16 @@ class SdCardFontManager {
   SdCardFontManager(const SdCardFontManager&) = delete;
   SdCardFontManager& operator=(const SdCardFontManager&) = delete;
 
-  // Load the font file matching fontSizeEnum (SMALL=0 .. EXTRA_LARGE=3) by
+  // Load the single size whose pointSize is closest to targetPtSize. Only one
-  // ordinal position in the family's sorted size list. Only one .cpfont file
+  // .cpfont file is loaded; other sizes remain on disk. This keeps resident
-  // is loaded; other sizes remain on disk. This keeps resident interval +
+  // interval + kern/ligature tables to one size's worth of memory.
-  // kern/ligature tables to one size's worth of memory.
+  //
+  // Closest-pt selection is robust against families that don't ship the
+  // canonical {12,14,16,18}: a family with only [10,14,18] still resolves
+  // any reasonable target, where ordinal slot-mapping by SMALL..EXTRA_LARGE
+  // would mis-select.
   // Returns true on success.
-  bool loadFamily(const SdCardFontFamilyInfo& family, GfxRenderer& renderer, uint8_t fontSizeEnum);
+  bool loadFamily(const SdCardFontFamilyInfo& family, GfxRenderer& renderer, uint8_t targetPtSize);
 
   // Unload everything, unregister from renderer.
   void unloadAll(GfxRenderer& renderer);

lib/EpdFont/SdCardFontRegistry.cpp

@@ -4,6 +4,8 @@
 #include <Logging.h>
 
 #include <algorithm>
+#include <climits>
+#include <cstdlib>
 #include <cstring>
 
 // --- SdCardFontFamilyInfo helpers ---
@@ -38,6 +40,21 @@ std::vector<uint8_t> SdCardFontFamilyInfo::availableSizes() const {
   return sizes;
 }
 
+const SdCardFontFileInfo* SdCardFontFamilyInfo::pickClosestSize(uint8_t targetPtSize) const {
+  const SdCardFontFileInfo* selected = nullptr;
+  int bestDiff = INT_MAX;
+  for (const auto& f : files) {
+    int diff = std::abs(static_cast<int>(f.pointSize) - static_cast<int>(targetPtSize));
+    // Strict < ensures the first scan wins on ties; then tie-break by smaller
+    // pointSize to make the choice independent of filesystem enumeration order.
+    if (diff < bestDiff || (diff == bestDiff && selected && f.pointSize < selected->pointSize)) {
+      bestDiff = diff;
+      selected = &f;
+    }
+  }
+  return selected;
+}
+
 // --- SdCardFontRegistry ---
 
 bool SdCardFontRegistry::parseFilename(const char* filename, uint8_t& size, uint8_t& style) {

lib/EpdFont/SdCardFontRegistry.h

@@ -20,6 +20,15 @@ struct SdCardFontFamilyInfo {
   const SdCardFontFileInfo* findFile(uint8_t size, uint8_t style = 0) const;
   bool hasSize(uint8_t size) const;
   std::vector<uint8_t> availableSizes() const;
+
+  // Pick the file whose pointSize is closest to targetPtSize. On ties (equal
+  // distance) prefers the smaller pointSize so behaviour is deterministic
+  // across SD card layouts. Returns nullptr when files is empty.
+  //
+  // Robust against families that don't ship the canonical {12,14,16,18} set:
+  // a family with only [10,14,18] resolves target 12 → 10 (or 14 on tie),
+  // target 16 → 14 or 18, etc., instead of mis-indexing by ordinal slot.
+  const SdCardFontFileInfo* pickClosestSize(uint8_t targetPtSize) const;
 };
 
 class SdCardFontRegistry {

src/SdCardFontSystem.cpp

@@ -5,10 +5,15 @@
 
 #include "CrossPointSettings.h"
 
-static uint8_t fontSizeEnumFromSettings() {
+// Map fontSize enum (SMALL=0, MEDIUM=1, LARGE=2, EXTRA_LARGE=3) to the point
+// sizes shipped with the built-in fonts. Used to drive closest-pt selection
+// in the SD card font registry (see SdCardFontFamilyInfo::pickClosestSize).
+static constexpr uint8_t FONT_SIZE_TO_PT[CrossPointSettings::FONT_SIZE_COUNT] = {12, 14, 16, 18};
+
+static uint8_t targetPtSizeFromSettings() {
   uint8_t e = SETTINGS.fontSize;
   if (e >= CrossPointSettings::FONT_SIZE_COUNT) e = 1;  // default to MEDIUM
-  return e;
+  return FONT_SIZE_TO_PT[e];
 }
 
 void SdCardFontSystem::begin(GfxRenderer& renderer) {
@@ -25,7 +30,7 @@ void SdCardFontSystem::begin(GfxRenderer& renderer) {
   if (SETTINGS.sdFontFamilyName[0] != '\0') {
     const auto* family = registry_.findFamily(SETTINGS.sdFontFamilyName);
     if (family) {
-      if (manager_.loadFamily(*family, renderer, fontSizeEnumFromSettings())) {
+      if (manager_.loadFamily(*family, renderer, targetPtSizeFromSettings())) {
         LOG_DBG("SDFS", "Loaded SD card font family: %s", SETTINGS.sdFontFamilyName);
       } else {
         LOG_ERR("SDFS", "Failed to load SD font family: %s (clearing)", SETTINGS.sdFontFamilyName);
@@ -53,7 +58,7 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
 
   const char* wantedFamily = SETTINGS.sdFontFamilyName;
   const std::string& currentFamily = manager_.currentFamilyName();
-  const uint8_t sizeEnum = fontSizeEnumFromSettings();
+  const uint8_t targetPt = targetPtSizeFromSettings();
 
   if (wantedFamily[0] == '\0') {
     if (!currentFamily.empty()) {
@@ -62,8 +67,8 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
     return;
   }
 
-  // Reload if family changed OR if the user-selected size maps to a
+  // Reload if family changed OR if the user-selected size now resolves to a
-  // different file than what's currently loaded OR if the registry was
+  // different on-disk file than what's currently loaded OR if the registry was
   // just rediscovered (file may have been replaced on disk).
   bool familyMatches = (currentFamily == wantedFamily);
   if (familyMatches) {
@@ -74,13 +79,11 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
       SETTINGS.sdFontFamilyName[0] = '\0';
       return;
     }
-    auto sizes = family->availableSizes();
+    const auto* best = family->pickClosestSize(targetPt);
-    uint8_t idx = sizeEnum;
+    const uint8_t bestPt = best ? best->pointSize : 0;
-    if (idx >= sizes.size()) idx = sizes.size() - 1;
+    if (!registryWasDirty && bestPt == manager_.currentPointSize()) return;
-    uint8_t wantedPt = sizes.empty() ? 0 : sizes[idx];
+    LOG_DBG("SDFS", "Reloading %s: size %u -> %u (target %u)%s", wantedFamily, manager_.currentPointSize(), bestPt,
-    if (!registryWasDirty && wantedPt == manager_.currentPointSize()) return;
+            targetPt, registryWasDirty ? " [registry dirty]" : "");
-    LOG_DBG("SDFS", "Reloading %s: size %u -> %u (enum %u)%s", wantedFamily, manager_.currentPointSize(), wantedPt,
-            sizeEnum, registryWasDirty ? " [registry dirty]" : "");
   }
 
   if (!currentFamily.empty()) {
@@ -89,7 +92,7 @@ void SdCardFontSystem::ensureLoaded(GfxRenderer& renderer) {
 
   const auto* family = registry_.findFamily(wantedFamily);
   if (family) {
-    if (manager_.loadFamily(*family, renderer, sizeEnum)) {
+    if (manager_.loadFamily(*family, renderer, targetPt)) {
       LOG_DBG("SDFS", "Loaded SD font family: %s", wantedFamily);
     } else {
       LOG_ERR("SDFS", "Failed to load SD font family: %s (clearing)", wantedFamily);