Better error message when seek fails

> hexyl "$(which hexyl)" --skip=-5gib
    Error: Failed to jump to the desired input position. This could be
    caused by a negative offset that is too large or by an input that is not
    seek-able (e.g. if the input comes from a pipe).

ref: #114

Cargo.lock

@@ -1,5 +1,14 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+[[package]]
+name = "aho-corasick"
+version = "0.7.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b476ce7103678b0c6d3d395dbbae31d48ff910bd28be979ba5d48c6351131d0d"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "ansi_term"
 version = "0.11.0"
@@ -48,6 +57,12 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "autocfg"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cdb031dd78e28731d87d56cc8ffef4a8f36ca26c38fe2de700543e627f8a464a"
+
 [[package]]
 name = "bitflags"
 version = "1.2.1"
@@ -81,6 +96,15 @@ version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fea41bba32d969b513997752735605054bc0dfa92b4c56bf1189f2e174be7a10"
 
+[[package]]
+name = "float-cmp"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1267f4ac4f343772758f7b1bdcbe767c218bbab93bb432acbf5162bbf85a6c4"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "hermit-abi"
 version = "0.1.13"
@@ -100,15 +124,43 @@ dependencies = [
  "atty",
  "clap",
  "libc",
+ "predicates",
  "thiserror",
 ]
 
+[[package]]
+name = "lazy_static"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
+
 [[package]]
 name = "libc"
 version = "0.2.71"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9457b06509d27052635f90d6466700c65095fdf75409b3fbdd903e988b886f49"
 
+[[package]]
+name = "memchr"
+version = "2.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0ee1c47aaa256ecabcaea351eae4a9b01ef39ed810004e298d2511ed284b1525"
+
+[[package]]
+name = "normalize-line-endings"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61807f77802ff30975e01f4f071c8ba10c022052f98b3294119f3e615d13e5be"
+
+[[package]]
+name = "num-traits"
+version = "0.2.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a64b1ec5cda2586e284722486d802acf1f7dbdc623e2bfc57e65ca1cd099290"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "predicates"
 version = "1.0.5"
@@ -116,7 +168,10 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "96bfead12e90dccead362d62bb2c90a5f6fc4584963645bc7f71a735e0b0735a"
 dependencies = [
  "difference",
+ "float-cmp",
+ "normalize-line-endings",
  "predicates-core",
+ "regex",
 ]
 
 [[package]]
@@ -153,6 +208,24 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "regex"
+version = "1.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "38cf2c13ed4745de91a5eb834e11c00bcc3709e773173b2ce4c56c9fbde04b9c"
+dependencies = [
+ "aho-corasick",
+ "memchr",
+ "regex-syntax",
+ "thread_local",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.6.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b181ba2dcf07aaccad5448e8ead58db5b742cf85dfe035e2227f137a539a189"
+
 [[package]]
 name = "strsim"
 version = "0.8.0"
@@ -210,6 +283,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "thread_local"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d40c6d1b69745a6ec6fb1ca717914848da4b44ae29d9b3080cbee91d72a69b14"
+dependencies = [
+ "lazy_static",
+]
+
 [[package]]
 name = "treeline"
 version = "0.1.0"

Cargo.toml

@@ -23,7 +23,8 @@ default-features = false
 features = ["suggestions", "color", "wrap_help"]
 
 [dev-dependencies]
-assert_cmd = "1.0.1"
+assert_cmd = "1.0"
+predicates = "1.0"
 
 [profile.release]
 lto = true

src/bin/hexyl.rs

@@ -150,11 +150,19 @@ fn run() -> Result<(), AnyhowError> {
 
     let skip_offset = if let Some(ByteOffset { kind, value }) = skip_arg {
         let value = value.into_inner();
-        reader.seek(match kind {
+        reader
+            .seek(match kind {
                 ByteOffsetKind::ForwardFromBeginning | ByteOffsetKind::ForwardFromLastOffset => {
                     SeekFrom::Current(value)
                 }
                 ByteOffsetKind::BackwardFromEnd => SeekFrom::End(value.checked_neg().unwrap()),
+            })
+            .map_err(|_| {
+                anyhow!(
+                    "Failed to jump to the desired input position. \
+                     This could be caused by a negative offset that is too large or by \
+                     an input that is not seek-able (e.g. if the input comes from a pipe)."
+                )
             })?
     } else {
         0

tests/integration_tests.rs

@@ -84,6 +84,27 @@ fn prints_warning_when_skipping_past_the_end() {
         );
 }
 
+#[test]
+fn can_handle_negative_skip_arguments() {
+    hexyl()
+        .arg("hello_world_elf64")
+        .arg("--color=never")
+        .arg("--skip=-1KiB")
+        .assert()
+        .success();
+}
+
+#[test]
+fn fails_if_negative_offset_is_too_large() {
+    hexyl()
+        .arg("hello_world_elf64")
+        .arg("--color=never")
+        .arg("--skip=-1MiB")
+        .assert()
+        .failure()
+        .stderr(predicates::str::contains("Failed to jump"));
+}
+
 #[test]
 fn fail_if_length_and_bytes_options_are_used_simultaneously() {
     hexyl()