Implement and test util::file_seek

include/osmium/util/file.hpp

@@ -222,6 +222,22 @@ namespace osmium {
             return static_cast<std::size_t>(offset);
         }
 
+        /**
+         * Set current offset into file.
+         *
+         * @param fd Open file descriptor.
+         * @param offset Desired absolute offset into the file
+         */
+        inline void file_seek(int fd, size_t offset) noexcept {
+#ifdef _MSC_VER
+            osmium::detail::disable_invalid_parameter_handler diph;
+            // https://msdn.microsoft.com/en-us/library/1yee101t.aspx
+            _lseeki64(fd, offset, SEEK_SET);
+#else
+            ::lseek(fd, offset, SEEK_SET);
+#endif
+        }
+
         /**
          * Check whether the file descriptor refers to a TTY.
          *

test/CMakeLists.txt

@@ -187,6 +187,7 @@ add_unit_test(io test_compression_factory)
 add_unit_test(io test_file_formats)
 add_unit_test(io test_nocompression)
 add_unit_test(io test_output_utils)
+add_unit_test(io test_file_seek)
 add_unit_test(io test_string_table)
 
 add_unit_test(io test_bzip2 ENABLE_IF ${BZIP2_FOUND} LIBS ${BZIP2_LIBRARIES})

test/t/io/test_file_seek.cpp

@@ -0,0 +1,62 @@
+#include "catch.hpp"
+
+#include "utils.hpp"
+
+#include <osmium/io/detail/read_write.hpp>
+#include <osmium/util/file.hpp>
+
+/**
+ * Can read and seek around in files.
+ */
+TEST_CASE("Seek and read in files") {
+    /* gzipped data contains very few repetitions in the binary file format,
+     * which makes it easy to identify any problems. */
+    int fd = osmium::io::detail::open_for_reading(with_data_dir("t/io/data.osm.gz"));
+    struct seek_expectation {
+        size_t offset;
+        unsigned char eight_bytes[8];
+    };
+    const seek_expectation expectations[] = {
+        { 0x00, {0x1f, 0x8b, 0x08, 0x08, 0x19, 0x4a, 0x18, 0x54} },
+        { 0x00, {0x1f, 0x8b, 0x08, 0x08, 0x19, 0x4a, 0x18, 0x54} }, /* repeat / jump back */
+        { 0x21, {0x56, 0xc6, 0x18, 0xc3, 0xea, 0x6d, 0x4f, 0xe0} }, /* unaligned */
+        { 0xb3, {0xcd, 0x0a, 0xe7, 0x8f, 0xde, 0x00, 0x00, 0x00} }, /* close to end */
+        { 0x21, {0x56, 0xc6, 0x18, 0xc3, 0xea, 0x6d, 0x4f, 0xe0} }, /* "long" backward jump */
+    };
+    for (const auto& expect : expectations) {
+        printf("Trying to seek to %lu\n", expect.offset);
+        char actual_eight_bytes[8] = {0, 0, 0, 0, 0, 0, 0, 0};
+        osmium::util::file_seek(fd, expect.offset);
+        bool did_actually_read = osmium::io::detail::read_exactly(fd, &actual_eight_bytes[0], 8);
+        REQUIRE(did_actually_read);
+        for (int i = 0; i < 8; ++i) {
+            REQUIRE(expect.eight_bytes[i] == static_cast<unsigned char>(actual_eight_bytes[i]));
+        }
+    }
+}
+
+TEST_CASE("Seek close to end of file") {
+    /* gzipped data contains very few repetitions in the binary file format,
+     * which makes it easy to identify any problems. */
+    int fd = osmium::io::detail::open_for_reading(with_data_dir("t/io/data.osm.gz"));
+    REQUIRE(osmium::util::file_size(with_data_dir("t/io/data.osm.gz")) == 187);
+    char actual_eight_bytes[8] = {1, 1, 1, 1, 1, 1, 1, 1};
+    osmium::util::file_seek(fd, 186);
+    auto actually_read = osmium::io::detail::reliable_read(fd, &actual_eight_bytes[0], 8);
+    REQUIRE(actually_read == 1);
+    REQUIRE(actual_eight_bytes[0] == 0);
+    REQUIRE(actual_eight_bytes[1] == 1);
+}
+
+TEST_CASE("Seek to exact end of file") {
+    /* gzipped data contains very few repetitions in the binary file format,
+     * which makes it easy to identify any problems. */
+    int fd = osmium::io::detail::open_for_reading(with_data_dir("t/io/data.osm.gz"));
+    REQUIRE(osmium::util::file_size(with_data_dir("t/io/data.osm.gz")) == 187);
+    char actual_eight_bytes[8] = {1, 1, 1, 1, 1, 1, 1, 1};
+    osmium::util::file_seek(fd, 187);
+    auto actually_read = osmium::io::detail::reliable_read(fd, &actual_eight_bytes[0], 8);
+    REQUIRE(actually_read == 0);
+    REQUIRE(actual_eight_bytes[0] == 1);
+    REQUIRE(actual_eight_bytes[1] == 1);
+}