add user defined functions: md5, sqrt and stddev

Cargo.toml

@@ -9,13 +9,15 @@ edition = "2018"
 debug = true
 
 [dependencies]
-rusqlite = "0.25.1"
+rusqlite = {version = "0.25.1", features = ["functions"]}
 csv="1.1"
 sqlparser = "0.9.0"
 uuid={version = "0.8", features = ["v4"]}
 log = "0.4.14"
 simple_logger="1.11.0"
 clap = "3.0.0-beta.2"
+md5 = "0.7.0"
+statistical = "1.0.0"
 
 [dev-dependencies]
 assert_cmd="0.10"

src/db/functions.rs

@@ -0,0 +1,42 @@
+use rusqlite::functions::{Context, Aggregate};
+use rusqlite::Result;
+use statistical::population_standard_deviation;
+
+pub(crate) fn calculate_md5(ctx: &Context) -> Result<String> {
+    assert_eq!(ctx.len(), 1, "called with unexpected number of arguments");
+    let str = ctx.get_raw(0).as_str()?;
+    let hash = md5::compute(str);
+    Ok(format!("{:x}", hash))
+}
+
+pub(crate) fn calculate_sqrt(ctx: &Context) -> Result<f64> {
+    assert_eq!(ctx.len(), 1, "called with unexpected number of arguments");
+    let arg = ctx.get_raw(0);
+    if let Ok(f64) = arg.as_f64() {
+        Ok(f64.sqrt())
+    } else {
+        let i64 = arg.as_i64()?;
+        Ok((i64 as f64).sqrt())
+    }
+}
+pub struct Stddev;
+
+impl Aggregate<Vec<f64>, Option<f64>> for Stddev {
+    fn init(&self, _: &mut Context<'_>) -> Result<Vec<f64>> {
+        Ok(vec!())
+    }
+
+    fn step(&self, ctx: &mut Context<'_>, stdev: &mut Vec<f64>) -> Result<()> {
+        let next = ctx.get::<f64>(0)?;
+        stdev.push(next);
+        Ok(())
+    }
+
+    fn finalize(&self, _: &mut Context<'_>, numbers: Option<Vec<f64>>) -> Result<Option<f64>> {
+        println!("{:?}", &numbers);
+        let stddev = numbers.map(|n| population_standard_deviation(&n, None));
+        println!("{:?}", stddev);
+        Ok(stddev)
+    }
+}
+