#430: Add compareFiles method

Author: DmitryNekrasov

core/tzdbOnFilesystem/src/internal/filesystem.kt

@@ -4,6 +4,7 @@
  */
 
 @file:OptIn(ExperimentalForeignApi::class, UnsafeNumber::class)
+
 package kotlinx.datetime.internal
 
 import kotlinx.cinterop.*
@@ -43,3 +44,41 @@ internal fun Path.tryTraverseDirectory(
     }
     return true
 }
+
+internal fun compareFiles(file1: String, file2: String): Boolean = memScoped {
+    try {
+        val stat1 = alloc<stat>()
+        val stat2 = alloc<stat>()
+
+        if (stat(file1, stat1.ptr) != 0 || stat(file2, stat2.ptr) != 0) return false
+        if (stat1.st_size != stat2.st_size) return false
+
+        val f1 = fopen(file1, "rb") ?: return false
+        val f2 = fopen(file2, "rb") ?: run { fclose(f1); return false }
+
+        try {
+            val bufferSize = 4096
+            val buffer1 = allocArray<ByteVar>(bufferSize)
+            val buffer2 = allocArray<ByteVar>(bufferSize)
+
+            while (true) {
+                val read1 = fread(buffer1, 1u, bufferSize.toULong(), f1).toInt()
+                val read2 = fread(buffer2, 1u, bufferSize.toULong(), f2).toInt()
+
+                if (read1 != read2) return false
+                if (read1 == 0) break  // EOF
+
+                for (i in 0..<read1.toInt()) {
+                    if (buffer1[i] != buffer2[i]) return false
+                }
+            }
+
+            return true
+        } finally {
+            fclose(f1)
+            fclose(f2)
+        }
+    } catch (_: Throwable) {
+        return false
+    }
+}