feat: floor price set to 0.06 USDFC/month per data set

For storage amounts below ~24.576 GiB, the rate will be 0.06 USDFC / month.
Above this amount the price is per-byte.

Closes: #319

Author: rvagg

service_contracts/abi/FilecoinWarmStorageService.abi.json

@@ -362,6 +362,11 @@
             "name": "epochsPerMonth",
             "type": "uint256",
             "internalType": "uint256"
+          },
+          {
+            "name": "minimumPricePerMonth",
+            "type": "uint256",
+            "internalType": "uint256"
           }
         ]
       }
@@ -1686,6 +1691,27 @@
       }
     ]
   },
+  {
+    "type": "error",
+    "name": "InsufficientFundsForMinimumRate",
+    "inputs": [
+      {
+        "name": "payer",
+        "type": "address",
+        "internalType": "address"
+      },
+      {
+        "name": "minimumRequired",
+        "type": "uint256",
+        "internalType": "uint256"
+      },
+      {
+        "name": "available",
+        "type": "uint256",
+        "internalType": "uint256"
+      }
+    ]
+  },
   {
     "type": "error",
     "name": "InvalidChallengeCount",

service_contracts/src/FilecoinWarmStorageService.sol

@@ -158,6 +158,7 @@ contract FilecoinWarmStorageService is
         uint256 pricePerTiBPerMonthWithCDN; // Price with CDN add-on (3 USDFC per TiB per month)
         IERC20 tokenAddress; // Address of the USDFC token
         uint256 epochsPerMonth; // Number of epochs in a month
+        uint256 minimumPricePerMonth; // Minimum monthly charge for any dataset size (0.06 USDFC)
     }
 
     // Used for announcing upgrades, packed into one slot
@@ -196,6 +197,7 @@ contract FilecoinWarmStorageService is
     uint256 private immutable STORAGE_PRICE_PER_TIB_PER_MONTH; // 2.5 USDFC per TiB per month without CDN with correct decimals
     uint256 private immutable CACHE_MISS_PRICE_PER_TIB_PER_MONTH; // .5 USDFC per TiB per month for CDN with correct decimals
     uint256 private immutable CDN_PRICE_PER_TIB_PER_MONTH; // .5 USDFC per TiB per month for CDN with correct decimals
+    uint256 private immutable MINIMUM_STORAGE_RATE_PER_MONTH; // 0.06 USDFC per month minimum pricing floor
 
     // Fixed lockup amounts for CDN rails
     uint256 private immutable DEFAULT_CDN_LOCKUP_AMOUNT; // 0.7 USDFC
@@ -330,6 +332,7 @@ contract FilecoinWarmStorageService is
         STORAGE_PRICE_PER_TIB_PER_MONTH = (5 * 10 ** TOKEN_DECIMALS) / 2; // 2.5 USDFC
         CACHE_MISS_PRICE_PER_TIB_PER_MONTH = (1 * 10 ** TOKEN_DECIMALS) / 2; // 0.5 USDFC
         CDN_PRICE_PER_TIB_PER_MONTH = (1 * 10 ** TOKEN_DECIMALS) / 2; // 0.5 USDFC
+        MINIMUM_STORAGE_RATE_PER_MONTH = (6 * 10 ** TOKEN_DECIMALS) / 100; // 0.06 USDFC minimum
 
         // Initialize the lockup constants based on the actual token decimals
         DEFAULT_CDN_LOCKUP_AMOUNT = (7 * 10 ** TOKEN_DECIMALS) / 10; // 0.7 USDFC
@@ -1189,7 +1192,8 @@ contract FilecoinWarmStorageService is
 
     /**
      * @notice Calculate all per-epoch rates based on total storage size
-     * @dev Returns storage, cache miss, and CDN rates per TiB per month
+     * @dev Returns storage, cache miss, and CDN rates per TiB per month.
+     * Storage rate includes minimum pricing floor.
      * @param totalBytes Total size of the stored data in bytes
      * @return storageRate The PDP storage rate per epoch
      * @return cacheMissRate The cache miss rate per epoch
@@ -1200,18 +1204,26 @@ contract FilecoinWarmStorageService is
         view
         returns (uint256 storageRate, uint256 cacheMissRate, uint256 cdnRate)
     {
-        storageRate = calculateStorageSizeBasedRatePerEpoch(totalBytes, STORAGE_PRICE_PER_TIB_PER_MONTH);
+        storageRate = _calculateStorageRate(totalBytes);
         cacheMissRate = calculateStorageSizeBasedRatePerEpoch(totalBytes, CACHE_MISS_PRICE_PER_TIB_PER_MONTH);
         cdnRate = calculateStorageSizeBasedRatePerEpoch(totalBytes, CDN_PRICE_PER_TIB_PER_MONTH);
     }
 
     /**
      * @notice Calculate the storage rate per epoch (internal use)
+     * @dev Implements minimum pricing floor and returns the higher of the natural size-based rate or the minimum rate.
      * @param totalBytes Total size of the stored data in bytes
      * @return The storage rate per epoch
      */
     function _calculateStorageRate(uint256 totalBytes) internal view returns (uint256) {
-        return calculateStorageSizeBasedRatePerEpoch(totalBytes, STORAGE_PRICE_PER_TIB_PER_MONTH);
+        // Calculate natural size-based rate
+        uint256 naturalRate = calculateStorageSizeBasedRatePerEpoch(totalBytes, STORAGE_PRICE_PER_TIB_PER_MONTH);
+
+        // Calculate minimum rate (floor price converted to per-epoch)
+        uint256 minimumRate = MINIMUM_STORAGE_RATE_PER_MONTH / EPOCHS_PER_MONTH;
+
+        // Return whichever is higher: natural rate or minimum rate
+        return naturalRate > minimumRate ? naturalRate : minimumRate;
     }
 
     /**
@@ -1319,7 +1331,8 @@ contract FilecoinWarmStorageService is
             pricePerTiBPerMonthNoCDN: STORAGE_PRICE_PER_TIB_PER_MONTH,
             pricePerTiBPerMonthWithCDN: STORAGE_PRICE_PER_TIB_PER_MONTH + CDN_PRICE_PER_TIB_PER_MONTH,
             tokenAddress: usdfcTokenAddress,
-            epochsPerMonth: EPOCHS_PER_MONTH
+            epochsPerMonth: EPOCHS_PER_MONTH,
+            minimumPricePerMonth: MINIMUM_STORAGE_RATE_PER_MONTH
         });
     }