Don't update generated .h file if its not changed.

This changes logic of slang-generate to detect whether the newly generated .h file is different from the existing file, and update the existing file only when the actual content has changed. This helps prevent visual studio from repetitively rebuilding the slang project due to the header file being updated on every build.

Author: csyonghe

tools/slang-generate/main.cpp

@@ -370,6 +370,42 @@ void usage(char const* appName)
     fprintf(stderr, "usage: %s <input>\n", appName);
 }
 
+char* readAllText(char const * fileName)
+{
+    FILE * f;
+    fopen_s(&f, fileName, "rb");
+    if (!f)
+    {
+        return "";
+    }
+    else
+    {
+        fseek(f, 0, SEEK_END);
+        auto size = ftell(f);
+        char * buffer = new char[size + 1];
+        memset(buffer, 0, size + 1);
+        fseek(f, 0, SEEK_SET);
+        fread(buffer, sizeof(char), size, f);
+        fclose(f);
+        return buffer;
+    }
+}
+
+void writeAllText(char const *srcFileName, char const* fileName, char* content)
+{
+    FILE * f = nullptr;
+    fopen_s(&f, fileName, "wb");
+    if (!f)
+    {
+        printf("%s(0): error G0001: cannot write file %s\n", srcFileName, fileName);
+    }
+    else
+    {
+        fwrite(content, 1, strlen(content), f);
+        fclose(f);
+    }
+}
+
 int main(
     int     argc,
     char**  argv)
@@ -416,8 +452,9 @@ int main(
 
     Node* node = readInput(input, inputEnd);
 
+    // write output to a temporary file first
     char outputPath[1024];
-    sprintf_s(outputPath, "%s.h", inputPath);
+    sprintf_s(outputPath, "%s.temp.h", inputPath);
 
     FILE* outputStream;
     fopen_s(&outputStream, outputPath, "w");
@@ -426,5 +463,17 @@ int main(
 
     fclose(outputStream);
 
+    // update final output only when content has changed
+    char outputPathFinal[1024];
+    sprintf_s(outputPathFinal, "%s.h", inputPath);
+
+    char * allTextOld = readAllText(outputPathFinal);
+    char * allTextNew = readAllText(outputPath);
+    if (strcmp(allTextNew, allTextOld) != 0)
+    {
+        writeAllText(inputPath, outputPathFinal, allTextNew);
+    }
+    remove(outputPath);
+
     return 0;
 }