rent (#5205)

sdk/src/rent.rs

@@ -0,0 +1,72 @@
+//! configuration for network rent
+
+#[derive(Serialize, Deserialize, PartialEq, Clone, Debug)]
+pub struct Rent {
+    /// Rental rate
+    pub lamports_per_byte_year: u64,
+
+    /// exemption threshold, in years
+    pub exemption_threshold: f64,
+}
+
+/// default rental rate in lamports/byte-year, based on:
+///  2^^34 lamports per Sol
+///  $1 per Sol
+///  $0.01 per megabyte day
+///  $3.65 per megabyte year
+pub const DEFAULT_LAMPORTS_PER_BYTE_YEAR: u64 = 17_179_869_184 / 100 * 365 / (1024 * 1024);
+
+/// default amount of time (in years) the balance has to include rent for
+pub const DEFAULT_EXEMPTION_THRESHOLD: f64 = 2.0;
+
+impl Default for Rent {
+    fn default() -> Self {
+        Self {
+            lamports_per_byte_year: DEFAULT_LAMPORTS_PER_BYTE_YEAR,
+            exemption_threshold: DEFAULT_EXEMPTION_THRESHOLD,
+        }
+    }
+}
+
+impl Rent {
+    /// minimum balance due for a given size Account::data.len()
+    pub fn minimum_balance(&self, data_len: usize) -> u64 {
+        let bytes = data_len as u64;
+        bytes * (self.exemption_threshold * self.lamports_per_byte_year as f64) as u64
+    }
+
+    /// whether a given balance and data_len would be exempt
+    pub fn is_exempt(&self, balance: u64, data_len: usize) -> bool {
+        balance >= self.minimum_balance(data_len)
+    }
+
+    /// rent due on account's data_len with balance
+    pub fn due(&self, balance: u64, data_len: usize, years_elapsed: f64) -> u64 {
+        if self.is_exempt(balance, data_len) {
+            0
+        } else {
+            let bytes = data_len as u64;
+            ((self.lamports_per_byte_year * bytes) as f64 * years_elapsed) as u64
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_due() {
+        let rent = Rent::default();
+
+        assert_eq!(rent.due(0, 1, 1.0), DEFAULT_LAMPORTS_PER_BYTE_YEAR);
+        assert_eq!(
+            rent.due(
+                DEFAULT_LAMPORTS_PER_BYTE_YEAR * DEFAULT_EXEMPTION_THRESHOLD as u64,
+                1,
+                1.0
+            ),
+            0
+        );
+    }
+}