Random code

content/code/seek-test.md

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+title = "Seek Test"
+date = 2023-08-07
+
+[taxonomies]
+tags = ["random", "example", "rust", "seek"]
++++
+
+How to use `.seek()` in Tokio `BufReader`s.
+
+<!-- more -->
+
+What was the issue:
+
+I needed to read a file that have a header, marked with `#` and then followed
+by the data itself; all the data is TSV (tab-separated-values). Note that there
+is just one header and one data; it is not expected to find more headers/header
+information after you start reading the data.
+
+The easy solution could be:
+
+1. Open file;
+2. Read all the lines till there is one that **doesn't** start with `#`;
+3. Close file;
+4. Open the file again;
+5. Skip all lines that start with `#`;
+6. Process the result.
+
+Because I didn't want to read part of the file again, I wanted to rewind the
+cursor and have only one open. The general idea would be:
+
+1. Open file.
+2. Read all the lines till there is one that **doesn't** start with `#`;
+3. Rewind the file reader the number of bytes of this line, thus returning to
+   the very start of it;
+4. Consider the header read; the next reads would always produce the data.
+
+Example souce file:
+
+```csv
+# This is a header
+# Each field is tab-separated.
+# But so is the data, so it is all good.
+# field1	field2	field3	field4
+0	1	2	3
+1	2	3	4	
+2	3	4	5
+3	4	5	6
+```
+
+And the file that actually reads it:
+
+```rust
+use std::io::SeekFrom;
+use std::path::PathBuf;
+
+use tokio::fs::File;
+use tokio::io::AsyncBufReadExt;
+use tokio::io::AsyncSeekExt;
+use tokio::io::BufReader;
+
+#[tokio::main(flavor = "current_thread")]
+async fn main() {
+    let file_name = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("resources")
+        .join("the_file.tsv");
+
+    let file = File::open(&file_name).await.unwrap();
+    let reader = BufReader::new(file);
+    // Here is why I can't seek after using `.lines()`:
+    // `.lines()` takes `self` and not `&self`.
+    let mut lines = reader.lines();
+
+    println!("Finding headers...");
+    while let Some(line) = lines.next_line().await.unwrap() {
+        println!("\tGot line: {}", line);
+        if !line.starts_with('#') {
+            println!("\t\tOops, headers are done!");
+            // XXX issue here:
+            // We are assuming "+1" 'cause that the `\n` character that `.lines()` "eat" on every
+            // read. But, on DOS files, it would be `\r\n`, or 2 bytes.
+            // Need to find out a way to figure the line ending before doing "1+" or "2+" here.
+            let bytes = (1 + line.bytes().len() as i64) * -1;
+            println!("\t\tMust rewind {bytes} positions...");
+
+            let mut inner = lines.into_inner(); // get back our BufReader
+            inner.seek(SeekFrom::Current(bytes)).await.unwrap();
+
+            lines = inner.lines(); // build a line reader from the rewinded Reader
+            break;
+        }
+    }
+
+    println!("Now it should be data...");
+    while let Some(line) = lines.next_line().await.unwrap() {
+        println!("\tGot line: {}", line);
+    }
+}
+```