Calculating equity positions

Cargo.toml

[dependencies]
csv = { git = "https://github.com/BurntSushi/rust-csv.git" } #"1.0.0-beta.6"
serde = "1.0.38"
serde_derive = "1.0.38"
chrono = { verstion = "0.4.2", features = ["serde"] }
decimal = "2.0.4"

main.rs

#[macro_use]
extern crate serde_derive;
extern crate chrono;
extern crate csv;
extern crate decimal;

use chrono::{Datelike, NaiveDate};
use csv::ReaderBuilder;
use csv::Trim;
use decimal::d128; // cuz finance with floating point is bad
use std::env;

#[derive(Debug, Deserialize)]
#[serde(rename_all = "PascalCase")]
struct Grant {
    issue: NaiveDate,
    amount: u32,
    exercise: d128,
}

#[derive(Default, Debug)]
struct Aggregate {
    total_vested: u32,
    total_issued: u32,
    total_vested_exercise: d128,
    total_granted_exercise: d128,
}

fn vested(g: &Grant, on: NaiveDate) -> u32 {
    // vesting cliff
    if g.issue > on {
        return 0;
    }

    // If days of month indicate full month didn't pass, we'll have to subract an extra month
    let extra = if g.issue.day() > on.day() { 1 } else { 0 };
    let months = (on.year() - g.issue.year()) as u32 * 12 + on.month() - g.issue.month() - extra;
    match months {
        m if m < 12 => 0,              // nothing vested
        m if m >= 48 => g.amount,      // fully vested
        m => m as u32 * g.amount / 48, // vested 1/48 per month
    }
}

fn main() {
    let path = env::args().skip(1).next().expect("Missing arg. Must specify CSV file containing grants");
    let mut rdr = ReaderBuilder::new()
        .trim(Trim::All)
        .from_path(path)
        .unwrap();
    let mut grants = Vec::new();
    for record in rdr.deserialize() {
        let record: Grant = record.unwrap();
        grants.push(record);
    }

    println!("Date, Percent Vested, Total Vested, Total Granted, Exercise Vested Cost, Exercise Granted Cost");
    for y in 2015..=2019 {
        for m in 1..=12 {
            let totals: Aggregate = grants.iter().fold(Aggregate::default(), |agg, ref g| {
                let mut agg = agg;
                let date = NaiveDate::from_ymd(y, m, 1);
                if date > g.issue {
                    let vested = vested(&g, date);
                    agg.total_vested += vested;
                    agg.total_issued += g.amount;
                    agg.total_vested_exercise += g.exercise * d128::from(vested);
                    agg.total_granted_exercise += g.exercise * d128::from(g.amount);
                }
                agg
            });
            if totals.total_issued > 0 {
                let percent = 100 * totals.total_vested / totals.total_issued;
                println!(
                    "{}-{}-01, {}%, {}, {}, {}, {}",
                    y, m, percent, totals.total_vested, totals.total_issued, totals.total_vested_exercise, totals.total_granted_exercise
                );
            }
        }
    }
}