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os: implement File.read_bytes_into_newline method for reading into a buffer, line by line (#10129)

This commit is contained in:
Sebastian Schicho 2021-05-21 12:18:08 +02:00 committed by GitHub
parent 87ded1784e
commit 04ea2824d3
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2 changed files with 101 additions and 0 deletions

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@ -17,6 +17,8 @@ fn C.fseeko(voidptr, u64, int) int
fn C._fseeki64(voidptr, u64, int) int
fn C.getc(voidptr) int
// open_file can be used to open or create a file with custom flags and permissions and returns a `File` object.
pub fn open_file(path string, mode string, options ...int) ?File {
mut flags := 0
@ -362,6 +364,44 @@ pub fn (f &File) read_bytes_at(size int, pos u64) []byte {
return arr[0..nreadbytes]
}
// read_bytes_into_newline reads from the beginning of the file into the provided buffer.
// Each consecutive call on the same file continues reading where it previously ended.
// A read call is either stopped, if the buffer is full, a newline was read or EOF.
pub fn (f &File) read_bytes_into_newline(mut buf []byte) ?int {
if buf.len == 0 {
panic(@FN + ': `buf.len` == 0')
}
newline := 10
mut c := 0
mut buf_ptr := 0
mut nbytes := 0
for (buf_ptr < buf.len) {
c = C.getc(f.cfile)
match c {
C.EOF {
if C.feof(f.cfile) != 0 {
return nbytes
}
if C.ferror(f.cfile) != 0 {
return error('file read error')
}
}
newline {
buf[buf_ptr] = byte(c)
nbytes++
return nbytes
}
else {
buf[buf_ptr] = byte(c)
buf_ptr++
nbytes++
}
}
}
return nbytes
}
// read_bytes_into fills `buf` with bytes at the given position in the file.
// `buf` *must* have length greater than zero.
// Returns the number of read bytes, or an error.

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@ -59,6 +59,67 @@ fn testsuite_end() ? {
assert !os.is_dir(tfolder)
}
// test_read_bytes_into_newline_text tests reading text from a file with newlines.
// This test simulates reading a larger text file step by step into a buffer and
// returning on each newline, even before the buffer is full, and reaching EOF before
// the buffer is completely filled.
fn test_read_bytes_into_newline_text() ? {
mut f := os.open_file(tfile, 'w') ?
f.write_string('Hello World!\nGood\r morning.') ?
f.close()
f = os.open_file(tfile, 'r') ?
mut buf := []byte{len: 8}
n0 := f.read_bytes_into_newline(mut buf) ?
assert n0 == 8
n1 := f.read_bytes_into_newline(mut buf) ?
assert n1 == 5
n2 := f.read_bytes_into_newline(mut buf) ?
assert n2 == 8
n3 := f.read_bytes_into_newline(mut buf) ?
assert n3 == 6
f.close()
}
// test_read_bytes_into_newline_binary tests reading a binary file with NUL bytes.
// This test simulates the scenario when a byte stream is read and a newline byte
// appears in that stream and an EOF occurs before the buffer is full.
fn test_read_bytes_into_newline_binary() ? {
os.rm(tfile) or {} // FIXME This is a workaround for macos, because the file isn't truncated when open with 'w'
mut bw := []byte{len: 15}
bw[9] = 0xff
bw[12] = 10 // newline
n0_bytes := bw[0..10]
n1_bytes := bw[10..13]
n2_bytes := bw[13..]
mut f := os.open_file(tfile, 'w') ?
f.write(bw) ?
f.close()
f = os.open_file(tfile, 'r') ?
mut buf := []byte{len: 10}
n0 := f.read_bytes_into_newline(mut buf) ?
assert n0 == 10
assert buf[..n0] == n0_bytes
n1 := f.read_bytes_into_newline(mut buf) ?
assert n1 == 3
assert buf[..n1] == n1_bytes
n2 := f.read_bytes_into_newline(mut buf) ?
assert n2 == 2
assert buf[..n2] == n2_bytes
f.close()
}
// test_read_eof_last_read_partial_buffer_fill tests that when reading a file
// the end-of-file is detected and results in a none error being returned. This
// test simulates file reading where the end-of-file is reached inside an fread