Replace MPTID with PathFindMPT in AssetCache.

Make line direction non-optional in getPathsOut().
Add Path unit-tests.

src/test/app/MPToken_test.cpp

@@ -3336,8 +3336,9 @@ class MPToken_test : public beast::unit_test::suite
                 find_paths(env, carol, dan, MPT(-1));
             BEAST_EXPECT(srcAmt == MPT1(100));
             BEAST_EXPECT(dstAmt == MPT(100));
-            // This path is consistent with IOU1/gw / IOU/gw path -
-            // [gw1, IOU/gw], except for gw1. This is due to no MPT rippling
+            // Has this been IOU path, it would have started with gw, but not
+            // MPT. This is due to MPT not being bidirectional unlike from
+            // trustline.
             BEAST_EXPECT(same(pathSet, stpath(IPE(mpt.issuanceID()))));
         }
 
@@ -3419,13 +3420,134 @@ class MPToken_test : public beast::unit_test::suite
                 find_paths(env, carol, dan, MPT1(-1));
             BEAST_EXPECT(srcAmt == MPT(100));
             BEAST_EXPECT(dstAmt == MPT1(100));
-            // This path is consistent with IOU/gw / IOU/gw2 -
-            // IOU/gw2 / IOU1/gw1 path -
-            // [gw, IOU2/gw2, IOU1/gw1], except for gw.
-            // This is due to no MPT rippling
+            // Has this been IOU path, it would have started with gw, but not
+            // MPT. This is due to MPT not being bidirectional unlike from
+            // trustline.
             BEAST_EXPECT(
                 same(pathSet, stpath(IPE(USD2), IPE(mpt1.issuanceID()))));
         }
+
+        // Cross-asset payment via offers (two steps)
+        // Start/End with mpt/mp2 and book steps in the middle
+        // offers are MPT/MPT
+        {
+            Env env = pathTestEnv(*this);
+            Account const gw2{"gw2"};
+            env.fund(XRP(1'000), gw, gw1, gw2, alice, bob, carol, dan);
+
+            MPTTester mpt(env, gw, {.holders = {alice, carol}, .fund = false});
+            mpt.create(
+                {.ownerCount = 1,
+                 .holderCount = 0,
+                 .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT = mpt["MPT"];
+            mpt.authorize({.account = alice});
+            mpt.authorize({.account = carol});
+            mpt.pay(gw, carol, 200);
+
+            MPTTester mpt1(env, gw1, {.holders = {bob, alice}, .fund = false});
+            mpt1.create(
+                {.ownerCount = 1, .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT1 = mpt1["MPT1"];
+            mpt1.authorize({.account = alice});
+            mpt1.pay(gw1, alice, 200);
+            mpt1.authorize({.account = bob});
+
+            MPTTester mpt2(env, gw2, {.holders = {bob, dan}, .fund = false});
+            mpt2.create(
+                {.ownerCount = 1, .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT2 = mpt2["MPT2"];
+            mpt2.authorize({.account = bob});
+            mpt2.pay(gw2, bob, 200);
+            mpt2.authorize({.account = dan});
+
+            env(offer(alice, MPT(100), MPT1(100)));
+            env(offer(bob, MPT1(100), MPT2(100)));
+            env.close();
+
+            auto const [pathSet, srcAmt, dstAmt] =
+                find_paths(env, carol, dan, MPT2(-1));
+            BEAST_EXPECT(srcAmt == MPT(100));
+            BEAST_EXPECT(dstAmt == MPT2(100));
+            // Has this been IOU path, it would have started with gw, but not
+            // MPT. This is due to MPT not being bidirectional unlike from
+            // trustline
+            BEAST_EXPECT(same(
+                pathSet,
+                stpath(IPE(mpt1.issuanceID()), IPE(mpt2.issuanceID()))));
+        }
+
+        // Cross-asset payment via offers (three steps)
+        // Start/End with mpt/mp3 and book steps in the middle
+        // offers are MPT/MPT
+        {
+            Env env = pathTestEnv(*this);
+            Account const gw2{"gw2"};
+            Account const gw3{"gw3"};
+            Account const greg{"greg"};
+            env.fund(
+                XRP(1'000), gw, gw1, gw2, gw3, alice, bob, carol, greg, dan);
+
+            MPTTester mpt(env, gw, {.holders = {alice, carol}, .fund = false});
+            mpt.create(
+                {.ownerCount = 1,
+                 .holderCount = 0,
+                 .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT = mpt["MPT"];
+            mpt.authorize({.account = alice});
+            mpt.authorize({.account = carol});
+            mpt.pay(gw, carol, 200);
+
+            MPTTester mpt1(env, gw1, {.holders = {bob, alice}, .fund = false});
+            mpt1.create(
+                {.ownerCount = 1, .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT1 = mpt1["MPT1"];
+            mpt1.authorize({.account = alice});
+            mpt1.pay(gw1, alice, 200);
+            mpt1.authorize({.account = bob});
+
+            MPTTester mpt2(env, gw2, {.holders = {bob, greg}, .fund = false});
+            mpt2.create(
+                {.ownerCount = 1, .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT2 = mpt2["MPT2"];
+            mpt2.authorize({.account = bob});
+            mpt2.pay(gw2, bob, 200);
+            mpt2.authorize({.account = greg});
+
+            MPTTester mpt3(env, gw3, {.holders = {greg, dan}, .fund = false});
+            mpt3.create(
+                {.ownerCount = 1, .flags = tfMPTCanTransfer | tfMPTCanTrade});
+            auto const MPT3 = mpt3["MPT3"];
+            mpt3.authorize({.account = greg});
+            mpt3.pay(gw3, greg, 200);
+            mpt3.authorize({.account = dan});
+
+            env(offer(alice, MPT(100), MPT1(100)));
+            env(offer(bob, MPT1(100), MPT2(100)));
+            env(offer(greg, MPT2(100), MPT3(100)));
+            env.close();
+
+            auto const [pathSet, srcAmt, dstAmt] =
+                find_paths(env, carol, dan, MPT3(-1));
+            BEAST_EXPECT(srcAmt == MPT(100));
+            BEAST_EXPECT(dstAmt == MPT3(100));
+            // Has this been IOU path, it would have started with gw, but not
+            // MPT. This is due to MPT not being bidirectional unlike from
+            // trustline
+            BEAST_EXPECT(same(
+                pathSet,
+                stpath(
+                    IPE(mpt1.issuanceID()),
+                    IPE(mpt2.issuanceID()),
+                    IPE(mpt3.issuanceID()))));
+
+            // TODO, path finding doesn't work, but the payment does
+            // Note that it doesn't work for all IOU either
+            env(pay(carol, dan, MPT3(100)),
+                path(~MPT1, ~MPT2, ~MPT3),
+                sendmax(MPT(100)),
+                txflags(tfNoRippleDirect | tfPartialPayment));
+        }
     }
 
     void

src/test/jtx/TestHelpers.h

@@ -174,7 +174,7 @@ rpf(jtx::Account const& src,
     jtx::Account const& dst,
     STAmount const& dstAmount,
     std::optional<STAmount> const& sendMax = std::nullopt,
-    std::optional<Currency> const& srcCurrency = std::nullopt);
+    std::optional<PathAsset> const& srcAsset = std::nullopt);
 
 jtx::Env
 pathTestEnv(beast::unit_test::suite& suite);

src/test/jtx/impl/TestHelpers.cpp

@@ -104,7 +104,7 @@ rpf(jtx::Account const& src,
     jtx::Account const& dst,
     STAmount const& dstAmount,
     std::optional<STAmount> const& sendMax,
-    std::optional<Currency> const& srcCurrency)
+    std::optional<PathAsset> const& srcAsset)
 {
     Json::Value jv = Json::objectValue;
     jv[jss::command] = "ripple_path_find";
@@ -113,11 +113,14 @@ rpf(jtx::Account const& src,
     jv[jss::destination_amount] = dstAmount.getJson(JsonOptions::none);
     if (sendMax)
         jv[jss::send_max] = sendMax->getJson(JsonOptions::none);
-    if (srcCurrency)
+    if (srcAsset)
     {
         auto& sc = jv[jss::source_currencies] = Json::arrayValue;
         Json::Value j = Json::objectValue;
-        j[jss::currency] = to_string(srcCurrency.value());
+        if (srcAsset->holds<Currency>())
+            j[jss::currency] = to_string(srcAsset->get<Currency>());
+        else
+            j[jss::mpt_issuance_id] = to_string(srcAsset->get<MPTID>());
         sc.append(j);
     }
 

src/xrpld/app/paths/AssetCache.cpp

@@ -129,21 +129,43 @@ AssetCache::getRippleLines(AccountID const& accountID, LineDirection direction)
     return it->second;
 }
 
-std::shared_ptr<std::vector<MPTID>> const&
+std::shared_ptr<std::vector<PathFindMPT>> const&
 AssetCache::getMPTs(const ripple::AccountID& account)
 {
     std::lock_guard sl(mLock);
 
     if (auto it = mpts_.find(account); it != mpts_.end())
         return it->second;
 
-    std::vector<MPTID> mpts;
+    std::vector<PathFindMPT> mpts;
     // Get issued/authorized tokens
     forEachItem(*ledger_, account, [&](std::shared_ptr<SLE const> const& sle) {
         if (sle->getType() == ltMPTOKEN_ISSUANCE)
-            mpts.push_back(makeMptID(sle->getFieldU32(sfSequence), account));
+        {
+            auto const mptID = makeMptID(sle->getFieldU32(sfSequence), account);
+            auto const maxAmount =
+                (*sle)[~sfMaximumAmount].value_or(maxMPTokenAmount);
+            bool const maxedOut = sle->at(sfOutstandingAmount) == maxAmount;
+            mpts.emplace_back(mptID, false, maxedOut);
+        }
         else if (sle->getType() == ltMPTOKEN)
-            mpts.push_back(sle->getFieldH192(sfMPTokenIssuanceID));
+        {
+            auto const mptID = sle->getFieldH192(sfMPTokenIssuanceID);
+            bool const zeroBalance = sle->at(sfMPTAmount) == 0;
+            bool const maxedOut = [&] {
+                if (auto const sleIssuance =
+                        ledger_->read(keylet::mptIssuance(mptID)))
+                {
+                    auto const maxAmount =
+                        (*sleIssuance)[~sfMaximumAmount].value_or(
+                            maxMPTokenAmount);
+                    return sleIssuance->at(sfOutstandingAmount) == maxAmount;
+                }
+                return true;
+            }();
+
+            mpts.emplace_back(mptID, zeroBalance, maxedOut);
+        }
     });
 
     totalMPTCount_ += mpts.size();
@@ -152,7 +174,8 @@ AssetCache::getMPTs(const ripple::AccountID& account)
         mpts_.emplace(account, nullptr);
     else
         mpts_.emplace(
-            account, std::make_shared<std::vector<MPTID>>(std::move(mpts)));
+            account,
+            std::make_shared<std::vector<PathFindMPT>>(std::move(mpts)));
 
     return mpts_[account];
 }

src/xrpld/app/paths/AssetCache.h

@@ -21,6 +21,7 @@
 #define RIPPLE_APP_PATHS_RIPPLELINECACHE_H_INCLUDED
 
 #include <xrpld/app/ledger/Ledger.h>
+#include <xrpld/app/paths/MPT.h>
 #include <xrpld/app/paths/TrustLine.h>
 #include <xrpl/basics/CountedObject.h>
 #include <xrpl/basics/hardened_hash.h>
@@ -62,7 +63,7 @@ class AssetCache final : public CountedObject<AssetCache>
     std::shared_ptr<std::vector<PathFindTrustLine>>
     getRippleLines(AccountID const& accountID, LineDirection direction);
 
-    std::shared_ptr<std::vector<MPTID>> const&
+    std::shared_ptr<std::vector<PathFindMPT>> const&
     getMPTs(AccountID const& account);
 
 private:
@@ -128,7 +129,7 @@ class AssetCache final : public CountedObject<AssetCache>
         AccountKey::Hash>
         lines_;
     std::size_t totalLineCount_ = 0;
-    hash_map<AccountID, std::shared_ptr<std::vector<MPTID>>> mpts_;
+    hash_map<AccountID, std::shared_ptr<std::vector<PathFindMPT>>> mpts_;
     std::size_t totalMPTCount_ = 0;
 };
 

src/xrpld/app/paths/MPT.h

@@ -0,0 +1,68 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2025 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef RIPPLE_APP_PATHS_MPT_H_INCLUDED
+#define RIPPLE_APP_PATHS_MPT_H_INCLUDED
+
+#include <xrpl/protocol/MPTIssue.h>
+
+namespace ripple {
+
+class PathFindMPT final
+{
+private:
+    MPTID const mptID_;
+    // If true then holder's balance is 0, always false for issuer
+    bool const zeroBalance_;
+    // OutstandingAmount is equal to MaximumAmount
+    bool const maxedOut_;
+
+public:
+    PathFindMPT(MPTID const& mptID)
+        : mptID_(mptID), zeroBalance_(false), maxedOut_(false)
+    {
+    }
+    PathFindMPT(MPTID const& mptID, bool zeroBalance, bool maxedOut)
+        : mptID_(mptID), zeroBalance_(zeroBalance), maxedOut_(maxedOut)
+    {
+    }
+    operator MPTID const&() const
+    {
+        return mptID_;
+    }
+    MPTID const&
+    getMptID() const
+    {
+        return mptID_;
+    }
+    bool
+    isZeroBalance() const
+    {
+        return zeroBalance_;
+    }
+    bool
+    isMaxedOut() const
+    {
+        return maxedOut_;
+    }
+};
+
+}  // namespace ripple
+
+#endif  // RIPPLE_APP_PATHS_MPT_H_INCLUDED

src/xrpld/app/paths/PathRequest.cpp

@@ -227,7 +227,7 @@ PathRequest::isValid(std::shared_ptr<AssetCache> const& crCache)
         if (auto mpts = crCache->getMPTs(*raDstAccount))
         {
             for (auto const& mpt : *mpts)
-                jvDestCur.append(to_string(mpt));
+                jvDestCur.append(to_string(mpt.getMptID()));
         }
 
         jvStatus[jss::destination_tag] =
@@ -564,7 +564,7 @@ PathRequest::findPaths(
             if (sourceAssets.size() >= RPC::Tuning::max_auto_src_cur)
                 return false;
             for (auto const& mpt : *mpts)
-                sourceAssets.insert(mpt);
+                sourceAssets.insert(MPTIssue{mpt});
         }
     }
 
@@ -737,7 +737,7 @@ PathRequest::doUpdate(
         if (auto mpts = cache->getMPTs(*raDstAccount))
         {
             for (auto const& mpt : *mpts)
-                destAssets.append(to_string(mpt));
+                destAssets.append(to_string(mpt.getMptID()));
         }
     }