make it work more oneshot like

src/main.rs

@@ -1,10 +1,10 @@
-use std::path::PathBuf;
 use std::io::Write;
-use std::time::{SystemTime, Duration};
+use std::path::PathBuf;
+use std::time::{Duration, SystemTime};
 
-use chrono::{DateTime, offset::Utc};
-use serde::{Deserialize, Serialize};
+use chrono::{offset::Utc, DateTime};
 use clap::Parser;
+use serde::{Deserialize, Serialize};
 
 const SECONDS_IN_MINUTE: u64 = 60;
 
@@ -68,7 +68,7 @@ enum PunchIn {
 
         /// The file to store time-tracking information.
         file: PathBuf,
-    }
+    },
 }
 
 #[derive(Serialize, Deserialize, Clone, Debug)]
@@ -82,7 +82,10 @@ fn main() {
     let args = PunchIn::parse();
 
     match args {
-        PunchIn::Start { file: path, round_up_to } => {
+        PunchIn::Start {
+            file: path,
+            round_up_to,
+        } => {
             let tt = if std::fs::exists(&path).is_ok_and(|v| v) {
                 let file = std::fs::File::open(&path).unwrap();
                 let mut tt: Vec<TimeTracking> = serde_json::from_reader(&file).unwrap();
@@ -106,36 +109,49 @@ fn main() {
             };
             println!("Writing to file: {tt:?}");
             std::fs::write(&path, serde_json::to_string(&tt).unwrap()).unwrap();
-        },
-        PunchIn::Report { format: ReportFormat::Markdown, timescale, file: path } => {
+        }
+        PunchIn::Report {
+            format: ReportFormat::Markdown,
+            timescale,
+            file: path,
+        } => {
             let header = timescale.header();
             println!("| Date       | {header} | Project |");
-            println!("|------------|-{}-|---------|", String::from("-").repeat(header.len()));
+            println!(
+                "|------------|-{}-|---------|",
+                String::from("-").repeat(header.len())
+            );
             let file = std::fs::File::open(&path).unwrap();
             let tt: Vec<TimeTracking> = serde_json::from_reader(&file).unwrap();
             for entry in tt {
                 let date = DateTime::<Utc>::from(entry.start);
-                println!("| {date} | {duration:>4}{timescale} | {project} |",
+                println!(
+                    "| {date} | {duration:>4}{timescale} | {project} |",
                     date = date.date_naive(),
                     duration = entry.end.as_secs() as f64 / timescale.divisor(),
                     project = entry.project,
                 );
             }
-        },
-        PunchIn::Report { format: ReportFormat::CSV, timescale, file: path } => {
+        }
+        PunchIn::Report {
+            format: ReportFormat::CSV,
+            timescale,
+            file: path,
+        } => {
             let header = timescale.header().to_lowercase();
             println!("date,{header},project");
             let file = std::fs::File::open(&path).unwrap();
             let tt: Vec<TimeTracking> = serde_json::from_reader(&file).unwrap();
             for entry in tt {
                 let date = DateTime::<Utc>::from(entry.start);
-                println!("{date},{duration}{timescale},{project}",
+                println!(
+                    "{date},{duration}{timescale},{project}",
                     date = date.date_naive(),
                     duration = entry.end.as_secs() as f64 / timescale.divisor(),
                     project = entry.project,
                 );
             }
-        },
+        }
     }
 }
 
@@ -144,42 +160,31 @@ fn start(tt: &mut Vec<TimeTracking>, round_up_to: u64) {
     let time_modulus = round_up_to * SECONDS_IN_MINUTE;
     let mut lines = std::io::stdin().lines();
     let stdout = std::io::stdout();
-    let mut last_time = SystemTime::now();
-    let mut last_project = String::new();
-    loop {
-        println!();
-        println!("Enter the project you're working on, or send EOL to terminate. When starting");
-        println!("work on a new project, or when sending EOL, your time will be locked in for");
-        println!("the current project");
+
     print!("Project: ");
     stdout.lock().flush().unwrap();
-        let mut has_line = false;
-        let line = lines.next();
-        // invariant: because line is never empty, last_project is only empty on first run
-        if !last_project.is_empty() {
-            // append the last project to the tt list
-            let elapsed_secs = last_time.elapsed().unwrap().as_secs();
+    let project = lines
+        .next()
+        .transpose()
+        .ok()
+        .flatten()
+        .filter(|l| !l.is_empty())
+        .unwrap();
+    let start_time = SystemTime::now();
+
+    print!("Press Enter to save the current project and elapsed time.");
+    stdout.lock().flush().unwrap();
+    lines.next();
+
+    // append the project to the tt list
+    let elapsed_secs = start_time.elapsed().unwrap().as_secs();
     // TODO: Rounding error: If this somehow gets to _exactly_ a round_up_to boundary, to
     // the second, it will round up. Is there a better math way to do this?
-            let rounded_secs = elapsed_secs + time_modulus -
-                ((elapsed_secs + time_modulus) % time_modulus);
+    let rounded_secs = elapsed_secs + time_modulus - ((elapsed_secs + time_modulus) % time_modulus);
     tt.push(TimeTracking {
-                start: last_time,
+        start: start_time,
         // Assume the human is not faster than the machine's ability to go back in time.
         end: Duration::from_secs(rounded_secs),
-                project: last_project.clone(),
+        project: project.clone(),
     });
 }
-        if let Some(Ok(line)) = line {
-            if !line.is_empty() {
-                has_line = true;
-                println!("Starting tracking for: {line}");
-                last_project = line;
-                last_time = SystemTime::now();
-            }
-        }
-        if !has_line {
-            break;
-        }
-    }
-}