From c23155790ad65896ec6d5a46c05a6813a969014c Mon Sep 17 00:00:00 2001
From: Enrico Lefass <M4SSD35TRUCT10N@users.noreply.github.com>
Date: Sun, 20 Oct 2019 19:45:16 +0200
Subject: [PATCH] os: add pub fn read_bytes and pub fn read_bytes_at

---
 vlib/os/os.v      | 21 +++++++++++++++++++++
 vlib/os/os_test.v | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/vlib/os/os.v b/vlib/os/os.v
index 1c58b7995f..77b040f0a4 100644
--- a/vlib/os/os.v
+++ b/vlib/os/os.v
@@ -71,6 +71,27 @@ fn C.ftell(fp voidptr) int
 fn C.getenv(byteptr) byteptr
 fn C.sigaction(int, voidptr, int)
 
+// read_bytes reads an amount of bytes from the beginning of the file
+pub fn (f File) read_bytes(size int) []byte {
+        return f.read_bytes_at(size, 0)
+}
+
+// read_bytes_at reads an amount of bytes at the given position in the file
+pub fn (f File) read_bytes_at(size, pos int) []byte {
+        mut data := malloc(size)
+        mut arr  := [`0`].repeat(size)
+
+        C.fseek(f.cfile, pos, C.SEEK_SET)
+        C.fread(data, 1, size, f.cfile)
+        C.fseek(f.cfile, 0, C.SEEK_SET)
+
+        for e := 0; e < size; e++ {
+                arr[e] = data[e]
+        }
+
+        return arr
+}
+
 // read_file reads the file in `path` and returns the contents.
 pub fn read_file(path string) ?string {
 	mode := 'rb'
diff --git a/vlib/os/os_test.v b/vlib/os/os_test.v
index f66322a86c..a0d64c35d1 100644
--- a/vlib/os/os_test.v
+++ b/vlib/os/os_test.v
@@ -33,6 +33,42 @@ fn test_write_and_read_string_to_file() {
   os.rm(filename)
 }
 
+// test_write_and_read_bytes checks for regressions made in the functions
+// read_bytes, read_bytes_at and write_bytes.
+fn test_write_and_read_bytes() {
+        file_name :=  './byte_reader_writer.tst'
+        payload   :=  [`I`, `D`, `D`, `Q`, `D`]
+
+        file_write := os.create(os.realpath(file_name)) or {
+                eprintln('failed to create file $file_name')
+                return
+        }
+
+        // We use the standard write_bytes function to write the payload and
+        // compare the length of the array with the file size (have to match).
+        file_write.write_bytes(payload.data, 5)
+
+        file_write.close()
+
+        assert payload.len == os.file_size(file_name)
+
+        file_read := os.open(os.realpath(file_name)) or {
+          eprintln('failed to open file $file_name')
+          return
+        }
+
+        // We only need to test read_bytes because this function calls
+        // read_bytes_at with second parameter zeroed (size, 0).
+        red_bytes := file_read.read_bytes(5)
+
+        file_read.close()
+
+        assert red_bytes.str() == payload.str()
+
+        // We finally delete the test file.
+        os.rm(file_name)
+}
+
 fn test_create_and_delete_folder() {
   folder := './test1'
   os.mkdir(folder)