Add gas cost

packages/vm/src/environment.rs

@@ -41,6 +41,14 @@ pub struct GasConfig {
     pub ed25519_batch_verify_cost: u64,
     /// ed25519 batch signature verification cost (single public key)
     pub ed25519_batch_verify_one_pubkey_cost: u64,
+    /// cost for reading memory regions <= 8MB
+    pub read_region_small_cost: LinearGasCost,
+    /// cost for reading memory regions > 8MB
+    pub read_region_large_cost: LinearGasCost,
+    /// cost for validating bytes into a String
+    pub string_from_bytes_cost: LinearGasCost,
+    /// cost for calling a host function
+    pub host_call_cost: u64,
 }
 
 impl Default for GasConfig {
@@ -58,10 +66,58 @@ impl Default for GasConfig {
             // From https://docs.rs/ed25519-zebra/2.2.0/ed25519_zebra/batch/index.html
             ed25519_batch_verify_cost: 63 * GAS_PER_US / 2,
             ed25519_batch_verify_one_pubkey_cost: 63 * GAS_PER_US / 4,
+            read_region_small_cost: LinearGasCost {
+                base: 200000,
+                per_item: 115,
+            },
+            read_region_large_cost: LinearGasCost {
+                base: 0,
+                per_item: 520,
+            },
+            string_from_bytes_cost: LinearGasCost {
+                base: 28700,
+                per_item: 1400,
+            },
+            host_call_cost: 18000,
+        }
+    }
+}
+
+impl GasConfig {
+    pub fn read_region_cost(&self, bytes: usize) -> VmResult<u64> {
+        const THRESHOLD: usize = 8 * 1000 * 1000;
+        if bytes <= THRESHOLD {
+            self.read_region_small_cost.total_cost(bytes as u64)
+        } else {
+            self.read_region_large_cost.total_cost(bytes as u64)
         }
     }
 }
 
+/// Linear gas cost model where the cost is linear in the number of items.
+///
+/// To calculate it, you sample the cost for a few different amounts of items and fit a line to it.
+/// Let `b` be that line of best fit. Then `base = b(0)` is the y-intercept and
+/// `per_item = b(1) - b(0)` the slope.
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub struct LinearGasCost {
+    /// This is a flat part of the cost, charged once per batch.
+    base: u64,
+    /// This is the cost per item in the batch.
+    per_item: u64,
+}
+
+impl LinearGasCost {
+    pub fn total_cost(&self, items: u64) -> VmResult<u64> {
+        self.total_cost_opt(items)
+            .ok_or_else(VmError::gas_depletion)
+    }
+
+    fn total_cost_opt(&self, items: u64) -> Option<u64> {
+        self.base.checked_add(self.per_item.checked_mul(items)?)
+    }
+}
+
 /** context data **/
 
 #[derive(Clone, PartialEq, Eq, Debug, Default)]