From 6e2499093b8437fad67fdb7570ef9cb207e7e51c Mon Sep 17 00:00:00 2001
From: epezent <epezent@rice.edu>
Date: Tue, 19 Jan 2021 19:42:22 -0600
Subject: [PATCH] add PrimUnreserve to AddTextVertical when the render
 characters is less than the expected characters (fixes utf8 issues)

---
 implot.cpp | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/implot.cpp b/implot.cpp
index 2ae66e1..c41cd40 100644
--- a/implot.cpp
+++ b/implot.cpp
@@ -282,17 +282,21 @@ static const ImPlotStyleVarInfo* GetPlotStyleVarInfo(ImPlotStyleVar idx) {
 //-----------------------------------------------------------------------------
 
 void AddTextVertical(ImDrawList *DrawList, ImVec2 pos, ImU32 col, const char *text_begin, const char* text_end) {
+    // the code below is based loosely on ImFont::RenderText
     if (!text_end)
         text_end = text_begin + strlen(text_begin);
     ImGuiContext& g = *GImGui;
     ImFont* font = g.Font;
-    pos.x = IM_FLOOR(pos.x); // + font->ConfigData->GlyphOffset.y);
-    pos.y = IM_FLOOR(pos.y); // + font->ConfigData->GlyphOffset.x);
-    const char* s = text_begin;
-    const int vtx_count = (int)(text_end - s) * 4;
-    const int idx_count = (int)(text_end - s) * 6;
-    DrawList->PrimReserve(idx_count, vtx_count);
+    // Align to be pixel perfect
+    pos.x = IM_FLOOR(pos.x);
+    pos.y = IM_FLOOR(pos.y);
     const float scale = g.FontSize / font->FontSize;
+    const char* s = text_begin;
+    int chars_exp = (int)(text_end - s);
+    int chars_rnd = 0;
+    const int vtx_count_max = chars_exp * 4;
+    const int idx_count_max = chars_exp * 6;
+    DrawList->PrimReserve(idx_count_max, vtx_count_max);
     while (s < text_end) {
         unsigned int c = (unsigned int)*s;
         if (c < 0x80) {
@@ -304,15 +308,20 @@ void AddTextVertical(ImDrawList *DrawList, ImVec2 pos, ImU32 col, const char *te
                 break;
         }
         const ImFontGlyph * glyph = font->FindGlyph((ImWchar)c);
-        if (glyph == NULL)
+        if (glyph == NULL) {
             continue;
+        }
         DrawList->PrimQuadUV(pos + ImVec2(glyph->Y0, -glyph->X0) * scale, pos + ImVec2(glyph->Y0, -glyph->X1) * scale,
                              pos + ImVec2(glyph->Y1, -glyph->X1) * scale, pos + ImVec2(glyph->Y1, -glyph->X0) * scale,
                              ImVec2(glyph->U0, glyph->V0), ImVec2(glyph->U1, glyph->V0),
                              ImVec2(glyph->U1, glyph->V1), ImVec2(glyph->U0, glyph->V1),
                              col);
         pos.y -= glyph->AdvanceX * scale;
+        chars_rnd++;
     }
+    // Give back unused vertices
+    int chars_skp = chars_exp-chars_rnd;
+    DrawList->PrimUnreserve(chars_skp*6, chars_skp*4);
 }
 
 double NiceNum(double x, bool round) {