diff --git a/Cargo.lock b/Cargo.lock
index a410a46..98951ce 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -338,6 +338,12 @@ version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fea41bba32d969b513997752735605054bc0dfa92b4c56bf1189f2e174be7a10"
 
+[[package]]
+name = "either"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "60b1af1c220855b6ceac025d3f6ecdd2b7c4894bfe9cd9bda4fbb4bc7c0d4cf0"
+
 [[package]]
 name = "env_filter"
 version = "0.1.0"
@@ -525,6 +531,15 @@ version = "1.70.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8478577c03552c21db0e2724ffb8986a5ce7af88107e6be5d2ee6e158c12800"
 
+[[package]]
+name = "itertools"
+version = "0.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "413ee7dfc52ee1a4949ceeb7dbc8a33f2d6c088194d9f922fb8318faf1f01186"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itoa"
 version = "1.0.11"
@@ -649,6 +664,7 @@ dependencies = [
  "clap_derive",
  "csv",
  "env_logger",
+ "itertools",
  "log",
  "predicates",
  "regex",
diff --git a/Cargo.toml b/Cargo.toml
index 63b6943..887075f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,7 @@ clap_derive = "4.5"
 log = "0.4"
 env_logger = "0.11.3"
 clap-verbosity-flag = "2.2"
+itertools = "0.13"
 anyhow = "1.0"
 chrono = { version = "0.4", features = ["serde"] }
 semver = { version = "1.0", features = ["serde"] }
diff --git a/README.md b/README.md
index 0bb5cbe..828d003 100644
--- a/README.md
+++ b/README.md
@@ -30,5 +30,6 @@ cargo doc --open
 - split into mods(maybe as lib crates)
 - add csv check/validate subcommand
 - skew measurement?
+- skew measurement test
 
 check `TODO`s in comments.
diff --git a/src/main.rs b/src/main.rs
index f15fa1e..8903cef 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -17,6 +17,8 @@ use semver::Version;
 use serde::{Deserialize, Serialize};
 use serde_with::{serde_as, DisplayFromStr};
 
+mod skew;
+
 /// PS Board QAQC shift related commands.
 #[derive(Parser, Debug)]
 #[command(version, about, long_about = None)]
@@ -520,7 +522,11 @@ fn main() -> Result<()> {
                 return Err(anyhow!("Invalid csv"));
             };
         }
-        Commands::AddSkew { logfile, outfile } => todo!("Not implemented"),
+        Commands::AddSkew { logfile, outfile } => {
+            let skew = skew::parse_count_file(logfile).context("Error during getting skew")?;
+            trace!("Skew: {}", skew);
+            todo!("Not implemented")
+        }
     }
     Ok(())
 }
diff --git a/src/skew.rs b/src/skew.rs
new file mode 100644
index 0000000..a47e8e0
--- /dev/null
+++ b/src/skew.rs
@@ -0,0 +1,78 @@
+use anyhow::{Context, Result};
+use itertools::{self, Itertools};
+use std::{
+    fs::File,
+    io::{BufRead, BufReader},
+    path::PathBuf,
+};
+
+fn parse_single_line(l: String) -> Result<(f64, u32, u32)> {
+    let mut splitted = l.split_whitespace();
+    let delay: f64 = splitted.next().context("No value found")?.parse()?;
+    let high_count: u32 = splitted.next().context("No value found")?.parse()?;
+    let low_count: u32 = splitted.next().context("No value found")?.parse()?;
+    assert!(splitted.next().is_none());
+    Ok((delay, high_count, low_count))
+}
+
+fn parse_counts(f: impl BufRead) -> Result<f64> {
+    let delay = f
+        .lines()
+        .filter_map(|l| parse_single_line(l.ok()?).ok())
+        .tuple_windows()
+        .find(|((_, high1, _), (_, high2, _))| *high1 < 500 && *high2 >= 500)
+        .context("Failed to get rise edge")?
+        .1
+         .0;
+    Ok(delay)
+}
+
+/// Read clock scan file and get skew.
+pub fn parse_count_file(filename: PathBuf) -> Result<f64> {
+    let buf_reader = BufReader::new(
+        File::open(filename.clone())
+            .context(format!("Error opening file {}", filename.display()))?,
+    );
+    parse_counts(buf_reader)
+}
+
+#[cfg(test)]
+mod test {
+    use super::parse_counts;
+    #[test]
+    fn test_parse_counts1() {
+        let file = "
+            0.1 0 1000
+            0.2 10 990
+            0.3 470 530
+            0.4 500 500
+            0.5 510 490
+            0.6 900 100
+            0.7 1000 0
+            "
+        .as_bytes();
+        assert_eq!(parse_counts(file).unwrap(), 0.4);
+        let file2 = "
+            0.1 0 1000
+            0.2 10 990
+            0.3 470 530
+            0.4 499 501
+            0.5 510 490
+            0.6 900 100
+            0.7 1000 0
+            "
+        .as_bytes();
+        assert_eq!(parse_counts(file2).unwrap(), 0.5);
+        let file3 = "
+            0.1 0 1000
+            0.2 10 990
+            0.3 470 530
+            0.4 501 499
+            0.5 510 490
+            0.6 900 100
+            0.7 1000 0
+            "
+        .as_bytes();
+        assert_eq!(parse_counts(file3).unwrap(), 0.4);
+    }
+}